Mood cheers up with doctor on department (10)
I believe the answer is:
atmosphere
'mood' is the definition.
(atmosphere is a kind of mood)
'with doctor on department' is the wordplay.
'with' means one lot of letters goes inside another.
'doctor' becomes 'mo' (medical officer).
'on' becomes 'at' (I've seen this in other clues).
'department' becomes 'sphere' (both can mean someone's area of expertise).
'at'+'sphere'='atsphere'
'mo' inserted within 'atsphere' is 'ATMOSPHERE'.
'cheers up' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
(Other definitions for atmosphere that I've seen before include "Air" , "We are always enveloped in this" , "Pervading tone or mood" , "character" , "Gas surrounding a planet" .)