Robber one up holding atomic weapon the wrong way (3-6)
I believe the answer is:
ram-raider
'robber' is the definition.
(I've seen this before)
'one up holding atomic weapon the wrong way' is the wordplay.
'one up' becomes 'rider' (I've seen this before).
'holding' means one lot of letters goes inside another.
'atomic' becomes 'a' (eg in A-test, atomic test).
'weapon' becomes 'arm' (synonyms).
'the wrong way' says the letters should be written backwards.
'arm' back-to-front is 'mra'.
'a'+'mra'='amra'
'rider' placed around 'amra' is 'RAM-RAIDER'.
(Other definitions for ram-raider that I've seen before include "One using vehicle to attack shop" , "Thief using vehicle to break into shops" .)