It’s only a pound single (5)
I believe the answer is:
alone
'it's only' is the definition.
('alone' can be a synonym of 'only')
'a pound single' is the wordplay.
'pound' becomes 'l'.
'single' becomes 'one' (I've seen this before).
'a'+'l'+'one'='ALONE'
(Other definitions for alone that I've seen before include "Without others being involved" , "Isolated from others" , "Without friends" , "unique" , "Exclusively; solitary" .)