Teacher's disapproving word over losing heart (5)
I believe the answer is:
tutor
'teacher's' is the definition.
(tutor is a kind of teacher)
'disapproving word over losing heart' is the wordplay.
'disapproving word' becomes 'tut' (I've seen this in another clue).
'losing heart' says to hollow out the word (remove centre letters).
'over' with its middle removed is 'or'.
'tut'+'or'='TUTOR'
(Other definitions for tutor that I've seen before include "Coach, teacher" , "Give individual instruction" , "Mentor" , "Edify" , "Private or university teacher" .)