Toff in uniform was in the army secretly (10)
I believe the answer is:
unobserved
'secretly' is the definition.
(without being observed)
'toff in uniform was in the army' is the wordplay.
'toff' becomes 'nob' (UK slang for a wealthy or upper-class person).
'in' means one lot of letters goes inside another.
'uniform' becomes 'U' (phonetic alphabet: alpha, bravo, charlie etc.).
'was in the army' becomes 'served' (served in the army).
'u'+'served'='userved'
'nob' inserted into 'userved' is 'UNOBSERVED'.
(Other definitions for unobserved that I've seen before include "behind closed doors" , "without supervision" .)