Hold onto cold fish (5)
I believe the answer is:
cling
'hold' is the definition.
(I've seen this before)
'onto cold fish' is the wordplay.
'onto' says to put letters next to each other.
'cold' becomes 'c' (eg on taps).
'fish' becomes 'ling' (ling is a kind of fish**).
'c' put next to 'ling' is 'CLING'.
(Other definitions for cling that I've seen before include "Be emotionally over-dependent" , "Hold on tightly, refuse to abandon" , "Stick close (to surface)" , "Hold on tightly, be dependent" , "Hold tightly, become attached" .)