Put at risk X after heart transplant (8)
I believe the answer is:
threaten
'put at risk' is the definition.
(I've seen this before)
'x after heart transplant' is the wordplay.
'x' becomes 'ten' (synonyms).
'after' says to put letters next to each other.
'transplant' indicates anagramming the letters.
'heart' with letters rearranged gives 'threa'.
'ten' after 'threa' is 'THREATEN'.
(Other definitions for threaten that I've seen before include "'Menace, intimidate (8)'" , "Warn of violence" , "Look likely to" , "Bully" , "Utter menaces" .)