For toll, knock back Gwyn? (5)
I believe the answer is:
knell
'for toll' is the definition.
(I've seen this before)
'knock back gwyn?' is the wordplay.
'back' suggests the final letters.
'gwyn?' becomes 'Nell' (Nell Gwyn, mistress of Charles II).
The last letter of 'knock' is 'k'.
'k'+'nell'='KNELL'
(Other definitions for knell that I've seen before include "Awesome ring" , "Funereal bell sound" , "Solemn warning" , "''The curfew tolls the ... of parting day'' (Thomas Gray)" , "Solemnly proclaim death" .)