Fish in school have disorderly retreat (3,5)
I believe the answer is:
sea trout
'fish' is the definition.
(sea trout is a kind of fish)
'school have disorderly retreat' is the wordplay.
'school' becomes 'S'.
'have' becomes 'eat' (eating is a kind of having).
'disorderly retreat' becomes 'rout' (I've seen this before).
's'+'eat'+'rout'='SEA-TROUT'
'in' is the link.
(Other definitions for sea trout that I've seen before include "Something fishy here" , "One angled" , "Salmon-like fish that also migrates from salt to fresh water" .)