Small skirt's pinned on back (5)
I believe the answer is:
minor
'small' is the definition.
(I know that small can be written as minor)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for minor that I've seen before include "Adolescent" , "Small-scale - child" , "Slight; little known" , "Of lesser importance" , "Smaller - child" .)