Winger mostly keen to hold line (5)
I believe the answer is:
eagle
'winger' is the definition.
(eagle is a type of bird)
'mostly keen to hold line' is the wordplay.
'mostly keen' becomes 'eage' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'to hold' is an insertion indicator.
'line' becomes 'l' (used when specifying particular lines from a poem).
'eage' placed around 'l' is 'EAGLE'.
Can you help me to learn more?
(Other definitions for eagle that I've seen before include "Erne, perhaps" , "The emblem of the US is bald" , "Valuable piece" , "A birdie (or better!)" , "Hook-billed bird of prey -- old US $10 coin" .)