One, out of condition, losing heart (4)
I believe the answer is:
unit
'one' is the definition.
('unit' can be a synonym of 'one')
'out of condition losing heart' is the wordplay.
'out of condition' becomes 'unfit' ('unfit' can be similar in meaning to 'out of condition').
'losing heart' says to hollow out the word (remove centre letters).
'unfit' with its middle removed is 'UNIT'.
(Other definitions for unit that I've seen before include "Separate part" , "Kitchen fitting" , "One part of a whole" , "Entity, system; component group" , "army group" .)