Happy with a smaller amount in sack (7)
I believe the answer is:
blessed
'happy' is the definition.
('blessed' can be similar in meaning to 'happy')
'a smaller amount in sack' is the wordplay.
'a smaller amount' becomes 'less' (I've seen this before).
'in' is an insertion indicator.
'sack' becomes 'bed' ('in the sack' means 'in bed').
'less' placed into 'bed' is 'BLESSED'.
'with' acts as a link.
(Other definitions for blessed that I've seen before include "darned" , "Happy; beatified" , "Bedless (anag.)" , "worthy of respect" , "Consecrated" .)