Finery on girl, hair barely done up (7)
I believe the answer is:
regalia
'finery' is the definition.
(synonyms)
'on girl hair barely done up' is the wordplay.
'on' means one lot of letters goes inside another (as in clothing 'on' a person).
'girl' becomes 'g'.
'barely' indicates the central letters (the word is bare of its external letters).
'done' becomes 'laer' (I can't explain this - if you can you should give a lot more credence to this answer).
'up' is a reversal indicator (in down clue: letters go upwards).
The central letters of 'hair' are 'ai'.
'ai'+'laer'='ailaer'
'g' going within 'ailaer' is 'ailager'.
'ailager' backwards is 'REGALIA'.
Can you help me to learn more?
(Other definitions for regalia that I've seen before include "Royal paraphernalia" , "Royal trappings" , "ceremonial gear" , "Badges of office" , "Crown, sceptre etc used at a coronation" .)