Sullen and cross in past (6)
I believe the answer is:
broody
'sullen' is the definition.
(thesaurus)
'cross in past' is the wordplay.
'cross' becomes 'rood' (term for a crucifix).
'in' means one lot of letters goes inside another.
'past' becomes 'by' (as in years gone by).
'rood' going within 'by' is 'BROODY'.
'and' acts as a link.
(Other definitions for broody that I've seen before include "Wanting eggs (or a baby)" , "Pensive" , "Thoughtful, unhappy" , "Morbidly thoughtful" , "Introspective" .)