Modified square containing 51 oddly depleted offices (9)
I believe the answer is:
qualified
'modified' is the definition.
(associated in meaning)
'square containing 51 oddly depleted offices' is the wordplay.
'square' becomes 'quad' (quad can mean a square space).
'containing' indicates putting letters inside.
'51' becomes 'LI' (Roman numerals).
'oddly depleted' means one should take alternating letters.
The alternate letters of 'offices' are 'fie'.
'li'+'fie'='lifie'
'quad' going around 'lifie' is 'QUALIFIED'.
(Other definitions for qualified that I've seen before include "guarded" , "Having credentials" , "Provided with the attributes necessary to do a job" , "Professional" , "Passed exams - with reservations" .)