Third-Ager on Scottish island (7)
I believe the answer is:
retiree
'third-ager' is the definition.
I can't judge whether this defines the answer.
'on scottish island' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to').
'scottish island' becomes 'tiree' (island of the Inner Hebrides).
're'+'tiree'='RETIREE'
Can you help me to learn more?
(Other definitions for retiree that I've seen before include "One put out to pasture" , "One having completed work" , "One ceasing to work" , "One no longer working" , "One stopping work" .)