Compelled number to join military unit (6)
I believe the answer is:
forced
'compelled' is the definition.
(forcing is a kind of compelling)
'number to join military unit' is the wordplay.
'number' becomes 'd' (D is an example**).
'to join' says to put letters next to each other.
'military unit' becomes 'force' ('force' can be a synonym of 'military unit').
'd' after 'force' is 'FORCED'.
(Other definitions for forced that I've seen before include "Unnatural; false" , "compulsory" , "Compelled, made to do something" , "Not natural" , "Like early rhubarb" .)