Ancient line in poem forgotten, finally (5)
I believe the answer is:
olden
'ancient' is the definition.
(thesaurus)
'line in poem forgotten finally' is the wordplay.
'line' becomes 'l'.
'in' means one lot of letters goes inside another.
'poem' becomes 'ode' (ode is a kind of poem**).
'finally' indicates one should take the final letters.
The final letter of 'forgotten' is 'n'.
'l' put inside 'ode' is 'olde'.
'olde'+'n'='OLDEN'
(Other definitions for olden that I've seen before include "Of past (times)" , "(Of days) in the long distant past!" , "of ancient history?" , "as in time gone by" , "Out of date" .)