Plain wicked pinching girl's rear (4)
I believe the answer is:
bald
'plain' is the definition.
(I've seen this before)
'wicked pinching girl's rear' is the wordplay.
'wicked' becomes 'bad' ('bad' can be similar in meaning to 'wicked').
'pinching' means one lot of letters goes inside another (taking inside).
'rear' says to take the final letters (the back of the word).
The final letter of 'girl' is 'l'.
'bad' going around 'l' is 'BALD'.
(Other definitions for bald that I've seen before include "Lacking hair on the scalp" , "Plain; hairless" , "Lacking hair on the head" , "Blunt; hairless" , "Without the natural covering" .)