Cold fish? Hold on! (5)
I believe the answer is:
cling
'hold on' is the definition.
(I know that hold on can be written as cling)
'cold fish?' is the wordplay.
'cold' becomes 'c' (eg on taps).
'fish?' becomes 'ling' (ling is a kind of fish).
'c'+'ling'='CLING'
(Other definitions for cling that I've seen before include "fast" , "Stick close (to surface)" , "Adhere, hold tight" , "Adhere to - ring" , "Hold on tightly, refuse to abandon" .)