One out of condition is losing heart (4)
I believe the answer is:
unit
'one' is the definition.
(synonyms)
'out of condition is losing heart' is the wordplay.
'out of condition' becomes 'unfit' (similar in meaning).
'is losing heart' means to remove the middle letters.
'unfit' with its centre removed is 'UNIT'.
(Other definitions for unit that I've seen before include "Single thing or person" , "Subdivision of a whole" , "Subdivision of a military group" , "Group that's part of a larger organisation" , "Single whole as part of a larger whole" .)