It is a bit tricky if you've never installed an amp or sub and you'll prob need to take a few hours to do it, but it isn't that hard really.
Main prob you've got if you have a standard headunit in the Clio is that there are no rca out sockets that you can simply plug a lead into, and then to your amp to give it the signal.
You can buy a speaker wire to RCA converter which solves this problem - mine cost about 10 quid from a wholesale place, but you can get them in most car shops. With mine you had to splice the wires on the converter into the left and right speaker wires. I did this behind the head unit, and used the rear speaker cables as it doesn't matter so much if they drain some of the power from the speaker. You then run RCA leads (red and white phono type) from the front of the car to your amp in the boot. That's the signal sorted.
Then you need to get power to the amp from the battery, and a signal to turn the amp on and off when your stereo is on or off. The power lead turned out to be a pain to get from the engine bay to the inside of the car, and I ended up drilling my own hole in the bulkhead, then putting a gromet in it to run the cable through. This takes some balls I'd say as you have to make sure you don't drill into any cables/wires/brake pipes! Once in the car it's easy to tuck the wire under the plastic trims on the inside of the car.
The remote lead (to switch the amp on and off) was spliced into one of the wires on the back of the stereo. You need a multi meter to work out which wire it is that gives a constant feed, and which one only a feed when stereo is on. In my 182 (04 plate) it was a pink wire.
So it's a bit of a fiddly job, but it sounds pretty good in my car - I didn't want to put an after market headunit in as:
a) cost more money to buy
b) lose the standard fit cd changer
c) you then have to take the face off your stereo everytime you get out the car.
Hope this helps!