Grub up — fish (no starter) (4)
I believe the answer is:
Here is my best explanation:
'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 "Inflict heavy defeat on", "clobber", "Whitewash", "Act like a pig", "Put to flight on tour", "Total(ly) defeat", "Disorderly retreat", "Big defeat", "Debacle" and "Overwhelming victory".)