Argue over number in diplomatic body (7)
I believe the answer is:
contend
'argue' is the definition.
(I know that argue can be written as contend)
'over number in diplomatic body' is the wordplay.
'over' becomes 'on' (physically on top of).
'number' becomes 'ten' (ten is a kind of number).
'in' indicates putting letters inside.
'diplomatic body' becomes 'CD' (abbreviation for corps diplomatique).
'on'+'ten'='onten'
'onten' placed within 'cd' is 'CONTEND'.
(Other definitions for contend that I've seen before include "Debate - contest - claim" , "Engage in a struggle" , "Tend NCO (anag.)" , "fight" , "Struggle; assert" .)