Wise, mostly courteous, and nice at heart (7)
I believe the answer is:
politic
'wise' is the definition.
(associated in meaning)
'mostly courteous and nice at heart' is the wordplay.
'mostly' means to remove the last letter.
'courteous' becomes 'polite' (similar in meaning).
'and' is a charade indicator (letters next to each other).
'at heart' means to look at the middle letters.
The middle of 'nice' is 'ic'.
'polite' with its final letter removed is 'polit'.
'polit'+'ic'='POLITIC'
(Other definitions for politic that I've seen before include "Prudent, discreet" , "Sensible, judicious" , "Intriguing" , "Expedient" , "Diplomatic" .)