Work a lot in being trained? It's not mandatory (8)
I believe the answer is:
Here is my best explanation:
'not mandatory' is the definition.
(I've seen this before)
'work a lot in being trained?' is the subsidiary indication.
'work' becomes 'op' (common abbreviation for 'opus'**).
'being trained?' indicates anagramming the letters.
'alotin' is an anagram of 'tional'.
'it's' is the link.
(Other definitions for optional that I've seen before include "A matter of choice", "Platoon I may leave to choice", "Open to choice - take your pick", "Too plain (anag.)", "Available as a choice but not obligatory", "Not obligatory", "'Left to choice, not compulsory (8)'", "Platoon I join as a matter of choice", "something extra?" and "Elective, as you choose".)