Plaintively express line in rhythm (5)
I believe the answer is:
bleat
'plaintively express' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'line in rhythm' is the wordplay.
'line' becomes 'l' (used when specifying particular lines from a poem).
'in' means one lot of letters goes inside another.
'rhythm' becomes 'beat' (synonyms).
'l' placed within 'beat' is 'BLEAT'.
Can you help me to learn more?
(Other definitions for bleat that I've seen before include "Complain; sheep's cry" , "Cry of a lamb at table" , "kid makes such a noise" , "Sound from the fold" , "Complaint from Cotswold" .)