Painter harboured by diplomat is secure (7)
I believe the answer is:
matisse
'painter' is the definition.
(I've seen this before)
'harboured by diplomat is secure' is the wordplay.
'harboured' says the answer is hidden in the clue (I've seen 'harbouring' mean this).
'MATISSE' can be found hidden inside 'by diplomat is secure'.
(Other definitions for matisse that I've seen before include "Henri ---, Twentieth Century painter" , "Mae sits badly for the French painter" , "Henri --, French artist" , "Henri -, Fr. artist" , "Henri ---, Fauvist painter" .)