What’s rude? Yell is, in church (8)
I believe the answer is:
churlish
'what's rude?' is the definition.
(thesaurus)
'yell is in church' is the wordplay.
'yell' becomes 'hurl' (as in hurling abuse at someone).
'in' indicates putting letters inside.
'church' becomes 'ch' (common abbreviation).
'hurl'+'is'='hurlis'
'hurlis' going inside 'ch' is 'CHURLISH'.
(Other definitions for churlish that I've seen before include "Mean-spirited and surly" , "Rude and surly" , "Rude and boorish" , "Ill-natured, boorish" , "Uncouth" .)