Regulars in navy, inspired by honour — it's a possibility (5)
I believe the answer is:
maybe
'it's a possibility' is the definition.
(something that may happen)
'regulars in navy inspired by honour' is the wordplay.
'regulars in' indicates alternate letters.
'inspired by' is an insertion indicator (some letters breathed in or 'inspired' by others).
'honour' becomes 'mbe' (Member of the British Empire award).
The alternating letters of 'navy' are 'ay'.
'ay' put inside 'mbe' is 'MAYBE'.
(Other definitions for maybe that I've seen before include "Perhaps, possibly" , "Perhaps, might happen" .)