Grub up — fish (no starter) (4)
I believe the answer is:
rout
'grub up' is the definition.
(both can mean to search for something in earth)
'fish no starter' is the wordplay.
'fish' becomes 'trout' (trout is a kind of fish).
'no starter' means to remove the first letter.
'trout' with its initial letter taken off is 'ROUT'.
(Other definitions for rout that I've seen before include "Debacle" , "Vanquish; thrash" , "Act like a pig" , "Completely defeat" , "clobber" .)