Determined tyro losing heart in fight (3-2)
I believe the answer is:
set to
'fight' is the definition.
(I know that set-to is a type of fight)
'determined tyro losing heart' is the wordplay.
'determined' becomes 'set' (similar in meaning).
'losing heart' means to remove the middle letters.
'tyro' with its middle removed is 'to'.
'set'+'to'='SET-TO'
'in' is the link.
(Other definitions for set to that I've seen before include "Scrap" , "You may see a fierce contest" , "Get cracking; start fighting" , "began working" , "Ding-dong" .)