Extravagant? Not in an army corps (5)
I believe the answer is:
outre
'extravagant?' is the definition.
(outre can mean extravagant or eccentric)
'not in an army corps' is the wordplay.
'not in' becomes 'out' ('out' is the opposite to 'in').
'an army corps' becomes 'RE' (Royal Engineers).
'out'+'re'='OUTRE'
(Other definitions for outre that I've seen before include "Eccentric or unseemly kind of route" , "Somewhat unusual" , "Oddball" , "Exotic" , "over the top" .)