Dirty picture upset Derek to begin with (7)
I believe the answer is:
obscene
'dirty' is the definition.
(similar in meaning)
'picture upset derek to begin with' is the wordplay.
'picture' becomes 'scene' (scene is a kind of picture).
'upset derek' becomes 'ob' (I am not sure about this - if you are sure you should believe this answer much more).
'to begin with' is a charade indicator (letters next to each other).
'scene' put after 'ob' is 'OBSCENE'.
Can you help me to learn more?
(Other definitions for obscene that I've seen before include "Offensively vulgar" , "Prurient" , "Lewd - offensive" , "Sexually offensive" , "X-rated" .)