Lover, one captured by pen in poetry? (5)
I believe the answer is:
swain
'lover' is the definition.
(old-fashioned term for a lover)
'one captured by pen in poetry?' is the wordplay.
'one captured by' is an insertion indicator.
'pen' becomes 'swan' (pen is a female swan).
'in poetry?' becomes 'i' (I am not sure about this - if you are sure you should believe this answer much more).
'swan' enclosing 'i' is 'SWAIN'.
Can you help me to learn more?
(Other definitions for swain that I've seen before include "lover of literature" , "Beau" , "Rustic suitor" , "Poetically, a young lover or suitor" , "Country boy" .)