Uncompromising senior officer being conveyed outside (12)
I believe the answer is:
intransigent
'uncompromising' is the definition.
(I've seen this before)
'senior officer being conveyed outside' is the wordplay.
'senior officer' becomes 'gen' (short for general, a senior army officer).
'being conveyed' becomes 'in transit' (in transit means on the move).
'outside' means one lot of letters goes inside another.
'gen' placed within 'intransit' is 'INTRANSIGENT'.
(Other definitions for intransigent that I've seen before include "unwilling to compromise" , "staying put" , "Obstinate" , "Not flexible" , "becomes immovable" .)