Arab stopping Mona's obsessive single-mindedness (9)
I believe the answer is:
monomania
'obsessive single-mindedness' is the definition.
(I've seen this before)
'arab stopping mona's' is the wordplay.
'arab' becomes 'omani' (Omani is an example).
'stopping' indicates putting letters inside (stopping can mean blocking a gap).
'omani' inserted within 'mona' is 'MONOMANIA'.
(Other definitions for monomania that I've seen before include "Single-minded passion" , "obsessive single-mindedness" , "Obsession with one thing" , "Unhealthy preoccupation with one thing" .)