Strange old doctor not practising medicine (3)
I believe the answer is:
odd
'strange' is the definition.
(I've seen this before)
'old doctor not practising medicine' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'doctor not practising medicine' becomes 'DD' (Doctor of Divinity).
'o'+'dd'='ODD'
(Other definitions for odd that I've seen before include "Not matching" , "Peculiar or eccentric" , "Bizarre - occasional" , "standing out from the crowd?" , "Not even? That's peculiar" .)