Not as much energy expended at first for tenant (6)
I believe the answer is:
lessee
'tenant' is the definition.
(I know that tenant can be written as lessee)
'not as much energy expended at first' is the wordplay.
'not as much' becomes 'less' (similar in meaning).
'at first' suggests taking the first letters.
The initial letters of 'energy expended' is 'ee'.
'less'+'ee'='LESSEE'
'for' acts as a link.
(Other definitions for lessee that I've seen before include "One holding lease on property" , "Who pays the rent" , "Tenant with a contract" , "The one taking out lease" , "Tenant who holds a lease" .)