Spirits are about a pound extra (6)
I believe the answer is:
morale
'spirits' is the definition.
(thesaurus)
'about a pound extra' is the wordplay.
'about' is an insertion indicator.
'pound' becomes 'l'.
'extra' becomes 'more' (I've seen this before).
'a'+'l'='al'
'al' inserted inside 'more' is 'MORALE'.
'are' acts as a link.
(Other definitions for morale that I've seen before include "Mood" , "Esprit, degree of confidence" , "Confidence, sense of wellbeing" , "Spirits" , "General level of confidence or optimism" .)