Besides, a pound’s nothing! (4)
I believe the answer is:
also
'besides' is the definition.
(thesaurus)
'a pound's nothing' is the wordplay.
'pound's' becomes 'ls'.
'nothing' becomes 'o' (looks like zero - 0).
'a'+'ls'+'o'='ALSO'
(Other definitions for also that I've seen before include "Furthermore" , "What's more" , "in accompaniment" , "revolutionary a part for Yugoslavia" , "As well, in addition" .)