To one fish it's drudgery (7)
I believe the answer is:
toiling
'it's drudgery' is the definition.
I can't judge whether this definition defines the answer.
'to one fish' is the wordplay.
I cannot quite see how this works, but
'to' is within the answer.
'one' could be 'i' (Roman numeral) and 'i' is located in the answer.
'fish' could be 'ling' (ling is a kind of fish) and 'ling' is located in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Another definition for toiling that I've seen is " Working long and hard".)