Many fish thus swallowing hard on a line (5)
I believe the answer is:
shoal
'many fish' is the definition.
(a shoal is a large number of fish)
'thus swallowing hard on a line' is the wordplay.
'thus' becomes 'so' (synonyms).
'swallowing' indicates putting letters inside.
'hard on' becomes 'h' (I can't justify this - if you can you should give a lot more credence to this answer).
'line' becomes 'l' (used when specifying particular lines from a poem).
'so' going around 'h' is 'sho'.
'sho'+'a'+'l'='SHOAL'
(Other definitions for shoal that I've seen before include "A great number of fish swimming together" , "Large group of fish - sandbank" , "Lots of fish" , "Fish grouped" , "Mob" .)