Lacking energy, attractive girl produces weapon (7)
I believe the answer is:
cutlass
'weapon' is the definition.
(cutlass is a kind of weapon)
'lacking energy attractive girl' is the wordplay.
'lacking' is a deletion indicator.
'energy' becomes 'e' (physics symbol as in E=mca).
'attractive' becomes 'cute' ('cute' can be similar in meaning to 'attractive').
'girl' becomes 'lass' (lass is a kind of girl**).
'cute' with 'e' taken away is 'cut'.
'cut'+'lass'='CUTLASS'
'produces' acts as a link.
(Other definitions for cutlass that I've seen before include "Curved, one-edged sword" , "Short broad sword" , "What Blackbeard wielded" , "Short, heavy curved sword" , "Sword formerly used by sailors" .)