The fish that gathers the school? (7)
I believe the answer is:
grouper
'the fish' is the definition.
(grouper is a kind of fish)
'gathers the school?' is the wordplay.
I cannot really understand how this works, but
'school?' could be 'group' (school is a kind of group) and 'group' is present in the answer.
The remaining letters 'er' is a valid word which might be clued in a way I don't see.
This may be the basis of clue (or it may be nonsense).
'that' acts as a link.
Can you help me to learn more?
(Other definitions for grouper that I've seen before include "A fish" , "that by implication isn't sole?" , "I order" .)