Possibly a yen to be held in honour (5)
I believe the answer is:
maybe
'possibly' is the definition.
('maybe' can be a synonym of 'possibly')
'a yen to be held in honour' is the wordplay.
'yen' becomes 'y'.
'to be held in' indicates putting letters inside.
'honour' becomes 'mbe' (**).
'a'+'y'='ay'
'ay' inserted within 'mbe' is 'MAYBE'.
(Other definitions for maybe that I've seen before include "Perhaps, possibly" , "Perhaps, might happen" .)