Happen to be coming in or going out? (6)
I believe the answer is:
betide
'happen to' is the definition.
(synonyms)
'be coming in or going out?' is the wordplay.
'coming in or going out?' becomes 'tide' (coming in or going out of the sea).
'be'+'tide'='BETIDE'
(Other definitions for betide that I've seen before include "Befall" , "Woe - -" , "Happen to (arch)" .)