Ruled though plainly no longer young (5)
I believe the answer is:
lined
'ruled' is the definition.
(I've seen this before)
'plainly no longer young' is the wordplay.
I cannot quite understand how this works, but
'no' could be 'n' (common abbreviation) and 'n' is found in the answer.
The remaining letters 'lied' 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).
'though' is the link.
Can you help me to learn more?
(Other definitions for lined that I've seen before include "What ruler did" , "Went along with" , "Covered inner surface" , "Strengthened" , "Given an inner covering" .)