Atmosphere in heart of mine became explosive (8)
I believe the answer is:
ambience
'atmosphere' is the definition.
(synonyms)
'heart of mine became explosive' is the wordplay.
'heart of' indicates the central letters.
'explosive' indicates an anagram.
The middle of 'mine' is 'in'.
'in'+'became'='inbecame'
'inbecame' with letters rearranged gives 'AMBIENCE'.
'in' is the link.
(Other definitions for ambience that I've seen before include "Suitable surroundings for manic bee" , "Aura (of a place)" , "Surrounding atmosphere" , "Character and atmosphere of a place" , "Mood of place" .)