Lacking principles, upset mother before exam (6)
I believe the answer is:
amoral
'lacking principles' is the definition.
(I've seen this before)
'upset mother before exam' is the wordplay.
'upset' is a reversal indicator.
'mother' becomes 'ma' (ma is a kind of mother).
'before' says to put letters next to each other.
'exam' becomes 'oral' (oral exam is a kind of exam).
'ma' in reverse letter order is 'am'.
'am'+'oral'='AMORAL'
(Other definitions for amoral that I've seen before include "Unscrupulous" , "Shameless" , "Lacking principles" , "without guiding principles" , "Unaffected by any sense of right and wrong" .)