Old girl often in distress? (6)
'old girl' is the definition.
'damsel' can be an answer for 'girl' (damsel is a kind of girl). I'm not certain of the 'old' bit.
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for damsel that I've seen before include "Maiden, often in distress", "Young single woman", "Young woman (archaic)", "An old girl, often in distress", "Maid".)