Many 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)
'many fish' is the wordplay.
'many' becomes 'c' (C is the Roman numeral for l00).
'fish' becomes 'ling' (ling is a kind of fish).
'c'+'ling'='CLING'
(Other definitions for cling that I've seen before include "Hold on as for dear life" , "Adhere, hold tight" , "Clasp closely" , "Hold on tightly, refuse to abandon" , "Stick close (to surface)" .)