Return obscene material at last, and think about it a lot (5)
I believe the answer is:
dwell
'think about it a lot' is the definition.
'return obscene material at last' is the wordplay.
'return' shows that the letters should be reversed in order.
'obscene' becomes 'lewd' ('lewd' can be a synonym of 'obscene').
'at last' suggests the final letters.
The last letter of 'material' is 'l'.
'lewd' reversed gives 'dwel'.
'dwel'+'l'='DWELL'
'and' is the link.
(Other definitions for dwell that I've seen before include "Live (in a place)" , "Settle" , "Think moodily (on something)" , "Focus attention (on)" , "Exist (within)" .)