Woman optionally holding the line (5)
I believe the answer is:
elsie
'woman' is the definition.
'optionally holding the line' is the wordplay.
I cannot quite see how this works, but
'line' could be 'el' (el is a kind of line) and 'el' is found within the answer.
The remaining letters 'sie' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for elsie that I've seen before include "Female name; rhymes with London football team" , "Name for girl" , "Girl's name" , "woman" , "Lady" .)