Soldier on a plain (9)
I believe the answer is:
persevere
'soldier on' is the definition.
(I've seen this before)
'a plain' is the wordplay.
'a' becomes 'per'.
'plain' becomes 'severe' ('severe' can be similar in meaning to 'plain').
'per'+'severe'='PERSEVERE'
(Other definitions for persevere that I've seen before include "Fight on" , "Persist, stick at" , "give it another try" , "Serve peer (anag.)" , "Continue regardless" .)