"Hang on!" from cold fish (5)
I believe the answer is:
cling
'hang on' is the definition.
(I've seen this in another clue)
'cold fish' is the wordplay.
'cold' becomes 'C' (eg on taps).
'fish' becomes 'ling' (ling is a kind of fish).
'c'+'ling'='CLING'
'from' acts as a link.
(Other definitions for cling that I've seen before include "Grasp urgently" , "Hold on tightly, be dependent" , "Keep hold" , "Hold tightly, become attached" , "Hold on as for dear life" .)