Cold fish behave like limpets (5)
I believe the answer is:
cling
'behave like limpets' is the definition.
(limpets cling to things)
'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 "Be emotionally over-dependent" , "Clasp closely" , "Stick close (to surface)" , "Adhere to - ring" , "Hold tightly, become attached" .)