Clue: Cask, one with coloured top (5)
Answer: TUNIC
Explanation: Definition: top
**TUN** ("cask") + **I** ("one") + **C**(oloured) 
*Hadn't come across "coloured" for C before, but my Chambers indicates it is (was?) a South African abbreviation – if that's the case, I don't want to see it used.