Privates and sentry occasionally cross (9)
I believe the answer is:
crotchety
'cross' is the definition.
(thesaurus)
'privates and sentry occasionally' is the wordplay.
'privates' becomes 'crotch' ('crotch' can be a synonym of 'privates').
'and' means one lot of letters go next to another.
'occasionally' means one should take alternating letters.
The alternate letters of 'sentry' are 'ety'.
'crotch'+'ety'='CROTCHETY'
(Other definitions for crotchety that I've seen before include "Irritable" , "Ill-tempered" , "irascible" , "Short-tempered" , "Bad-humoured or peevish" .)