Seeing that free heart was true (7)
I believe the answer is:
sincere
'true' is the definition.
(synonyms)
'seeing that free heart' is the wordplay.
'seeing that' becomes 'since' (both can mean 'when considering...').
'heart' means to look at the middle letters.
The central letters of 'free' are 're'.
'since'+'re'='SINCERE'
'was' is the link.
(Other definitions for sincere that I've seen before include "'Genuine, not deceitful (7)'" , "Free from pretence" , "Showing genuine feelings" , "Candid" , "Free from deceit" .)