One holding the line? (4)
I believe the answer is:
reel
'one' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'holding the line?' is the wordplay.
'holding' becomes 'ree' (I can't justify this - if you can you should believe this answer much more).
'the line?' becomes 'l' (used when specifying particular lines from a poem).
'ree'+'l'='REEL'
Can you help me to learn more?
(Other definitions for reel that I've seen before include "Lively dance or fishing rod controller" , "Dance; feel giddy" , "The angler pulls in his line using this" , "Highland dance" , "Dance - sway - cotton holder" .)