Trendy coach loses heart and thus becomes natural (9)
I believe the answer is:
intrinsic
'natural' is the definition.
(I've seen this before)
'trendy coach loses heart and thus' is the wordplay.
'trendy' becomes 'in' ('in' can mean fashionable).
'coach' becomes 'train' (both can mean to instruct a sportsperson).
'loses heart' says to hollow out the word (remove centre letters).
'and' means one lot of letters go next to another.
'thus' becomes 'sic' ('thus' in Latin).
'train' with its centre taken out is 'trin'.
'in'+'trin'+'sic'='INTRINSIC'
'becomes' is the link.
(Other definitions for intrinsic that I've seen before include "Natural" , "Belonging by its very nature" , "Naturally belonging" , "Of the essential nature of something" , "native?" .)