Transplant large number south of key (5)
I believe the answer is:
graft
'transplant' is the definition.
(synonyms)
'large number south of key' is the wordplay.
'large number' becomes 'raft' (I've seen this in another clue).
'south of' means one lot of letters go next to another (in a down clue, some letters go under others).
'key' becomes 'g' (musical key).
'raft' put after 'g' is 'GRAFT'.
(Other definitions for graft that I've seen before include "Transplant - hard work" , "Labour" , "Surgical transplant" , "Hard work; bribery" , "Hard work; corruption" .)