Alert mobster discharges megaton against household pest (9)
I believe the answer is:
observant
'alert' is the definition.
(alert can mean observant or vigilant)
'mobster discharges megaton against household pest' is the wordplay.
I cannot quite understand how this works, but
'against' could be 'v' (abbreviation for 'versus') and 'v' is present in the answer.
'household pest' could be 'ant' (I've seen this in another clue) and 'ant' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for observant that I've seen before include "aware of surroundings" , "Alert" , "Perceptive" , "attentive to detail" , "taking things in" .)