Girl showing staunch character, mostly (6)
I believe the answer is:
damsel
'girl' is the definition.
(damsel is a kind of girl)
'staunch character mostly' is the wordplay.
'staunch' becomes 'dam' (both can mean to block or stem).
'character' becomes 'self'.
'mostly' means to remove the last letter.
'self' with its final letter taken off is 'sel'.
'dam'+'sel'='DAMSEL'
'showing' is the link.
(Other definitions for damsel that I've seen before include "Young lady of old, perhaps in distress" , "Young girl" , "Old-fashioned young woman" , "little fly" , "An old girl, often in distress" .)