After losing heart, gained miss (4)
I believe the answer is:
need
'miss' is the definition.
(I've seen this before)
'after losing heart gained' is the wordplay.
'after losing heart' means to remove the middle letters.
'gained' becomes 'netted' (netting is a kind of gaining).
'netted' with its middle taken out is 'NEED'.
(Other definitions for need that I've seen before include "Rely on" , "Penury" , "Shortage" , "state of distress" , "state requiring relief" .)