Continued to protect air force (10)
I believe the answer is:
constraint
'force' is the definition.
(I've seen this before)
'continued to protect air' is the wordplay.
'continued' becomes 'cont' (common abbreviation).
'to protect' means one lot of letters goes inside another.
'air' becomes 'strain' (synonyms).
'cont' placed around 'strain' is 'CONSTRAINT'.
(Other definitions for constraint that I've seen before include "force" , "Limitation" , "Restriction - in contrast (anag)" , "restriction made" , "Check" .)