Doctor to Arab trapped in an upsetting obsession (9)
I believe the answer is:
monomania
'obsession' is the definition.
(I've seen this before)
'doctor to arab trapped in an upsetting' is the wordplay.
'doctor' becomes 'mo' (medical officer).
'to arab' becomes 'Omani' (someone from Oman).
'trapped in' is an insertion indicator.
'upsetting' shows that the letters should be reversed in order.
'an' in reverse letter order is 'na'.
'omani' inserted within 'na' is 'nomania'.
'mo'+'nomania'='MONOMANIA'
(Other definitions for monomania that I've seen before include "Obsession with one thing" , "Unhealthy preoccupation with one thing" , "Single-minded passion" , "obsessive single-mindedness" .)