Irritable fish keeps dry (5)
I believe the answer is:
ratty
'irritable' is the definition.
(I've seen this before)
'fish keeps dry' is the wordplay.
'fish' becomes 'ray' (ray is a kind of fish).
'keeps' means one lot of letters goes inside another (inserted letters are kept inside).
'dry' becomes 'tt' (short for teetotal - no alcohol).
'ray' placed around 'tt' is 'RATTY'.
(Other definitions for ratty that I've seen before include "describing vermin" , "Bad-tempered, irritable as a rodent" , "Waspish" , "In a bad temper" , "infestation of vermin" .)