A man has to observe the tenant (6)
I believe the answer is:
lessee
'the tenant' is the definition.
(lessee is a kind of tenant)
'a man has to observe' is the wordplay.
'a man' becomes 'les' ().
'has' is a charade indicator (letters next to each other).
'to observe' becomes 'see' (observe can mean to see something).
'les'+'see'='LESSEE'
(Other definitions for lessee that I've seen before include "Person who holds building on contract for specified time" , "I have to deal with letter" , "Tenant with a contract" , "Person who rents" , "One who rents" .)