Landlord to observe in secret (9)
I believe the answer is:
innkeeper
'landlord' is the definition.
(I've seen this before)
'observe in secret' is the wordplay.
'observe' becomes 'keep' ('keep' can be a synonym of 'observe').
'in' means one lot of letters goes inside another.
'secret' becomes 'inner' (I've seen this before).
'keep' going inside 'inner' is 'INNKEEPER'.
'to' acts as a link.
(Other definitions for innkeeper that I've seen before include "Job" , "Publican" , "For example, Mistress Quickly" , "Pub runner" , "Tavern owner" .)