Groom tries again, completely forgetting one line, in final run-through (5,9)
I believe the answer is:
dress rehearsal
'final run-through' is the definition.
(I've seen this before)
'groom tries again completely forgetting one line' is the wordplay.
'groom' becomes 'dress' ('dress' can be a synonym of 'groom').
'tries again' becomes 'rehears' ('hears' can be a synonym for 'tries' so perhaps 'rehears' is OK for 'tries again').
'completely forgetting' becomes 'a' (I can't justify this - if you can you should believe this answer much more).
'one line' becomes 'l'.
'dress'+'rehears'+'a'+'l'='DRESS REHEARSAL'
'in' is the link.
Can you help me to learn more?
(Other definitions for dress rehearsal that I've seen before include "Strip show doesn't need such" , "last chance to get things right" , "trial run" , "last practice" , "Final play run-through" .)