Shark needs to follow trawl (7)
I believe the answer is:
dogfish
'shark' is the definition.
(dogfish is a kind of shark)
'to follow trawl' is the wordplay.
'to follow' becomes 'dog' (I've seen this before).
'trawl' becomes 'fish' (I've seen this before).
'dog'+'fish'='DOGFISH'
'needs' acts as a link.
(Other definitions for dogfish that I've seen before include "'Small shark, sounds canine (7)'" , "Nursehound" , "Small, edible shark" , "Restaurants call it rock salmon" .)