It may not catch fish by the gross (3)
I believe the answer is:
net
'the gross' is the definition.
(thesaurus)
'it may not catch fish' is the wordplay.
I cannot really understand how this works, but
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is located in the answer.
The remaining letters 'ne' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'by' acts as a link.
Can you help me to learn more?
(Other definitions for net that I've seen before include "Mesh for the bottom line?" , "Lattice" , "Open-meshed fabric" , "Open mesh; catch" , "Fine open-weave fabric" .)