Aggressive, but the opposite when backup goes missing (5)
I believe the answer is:
pushy
'aggressive' is the definition.
(synonyms)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for pushy that I've seen before include "Excessively self-assertive" , "Overly assertive" , "Self-assertive, forceful" , "Aggressive; overbearing" , "Socially aggressive" .)