Soldier on exercise, Spartan, crushing resistance (9)
I believe the answer is:
persevere
'soldier on' is the definition.
(I've seen this before)
'exercise spartan crushing resistance' is the wordplay.
'exercise' becomes 'pe' (Physical Education).
'spartan' becomes 'severe' (I can't justify this - if you can you should give a lot more credence to this answer).
'crushing' is an insertion indicator (inserted letters are crushed by others).
'resistance' becomes 'r' (electronics abbreviation).
'pe'+'severe'='pesevere'
'pesevere' enclosing 'r' is 'PERSEVERE'.
Can you help me to learn more?
(Other definitions for persevere that I've seen before include "Continue in adversity" , "Keep going" , "hold on" , "Continue regardless" , "Serve peer (anag.)" .)