Work a lot in being trained? It's not mandatory (8)
I believe the answer is:
optional
'not mandatory' is the definition.
(I've seen this before)
'work a lot in being trained?' is the wordplay.
'work' becomes 'op' (abbreviation for opus**).
'being trained?' indicates anagramming the letters (to train can mean to grow something into a shape).
'a'+'lot'+'in'='alotin'
'alotin' with letters rearranged gives 'tional'.
'op'+'tional'='OPTIONAL'
'it's' is the link.
(Other definitions for optional that I've seen before include "Elective, as you choose" , "Platoon I may leave to choice" , "Unforced" , "Too plain (anag.)" , "something extra?" .)