Part seen occasionally in 'stinker', mag pens in review (7)
I believe the answer is:
segment
'part' is the definition.
(segmenting is a kind of parting)
'seen occasionally in stinker mag pens in review' is the wordplay.
'seen occasionally in' indicates alternate letters (letters selected at regular intervals).
'in review' is a reversal indicator.
'stinker'+'mag'+'pens'='stinkermagpens'
The alternate letters of 'stinkermagpens' are 'tnemges'.
'tnemges' reversed gives 'SEGMENT'.
(Other definitions for segment that I've seen before include "Part of, eg, an orange" , "Division" , "split up" , "Portion" , "One part of a whole" .)