In bed, I use less energy for sex (6)
I believe the answer is:
coitus
'sex' is the definition.
(coitus is a kind of sex)
'in bed i use less energy' is the wordplay.
'in' means one lot of letters goes inside another.
'bed' becomes 'cot' (cot is a type of bed).
'less' suggests deleting specific letters (less can mean minus).
'energy' becomes 'e' (physics symbol as in E=mc2).
'use' with 'e' taken out is 'us'.
'cot' placed around 'i' is 'coit'.
'coit'+'us'='COITUS'
'for' is the link.
(Other definitions for coitus that I've seen before include "congress" , "Sexual intercourse" .)