Kind heart of friend for Sally (6)
I believe the answer is:
sortie
'sally' is the definition.
(synonyms)
'kind heart of friend' is the wordplay.
'kind' becomes 'sort' (synonyms).
'heart of' indicates the central letters.
The central letters of 'friend' are 'ie'.
'sort'+'ie'='SORTIE'
'for' acts as a link.
(Other definitions for sortie that I've seen before include "Short trip; attack" , "Warplane's flight" , "Military raid" , "Military aircraft's flight" , "Sally" .)