Very small American male in charge (6)
I believe the answer is:
atomic
'very small' is the definition.
(I've seen this in another clue)
'american male in charge' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'male' becomes 'tom' (I've seen this in other clues).
'in charge' becomes 'ic' (abbreviation for 'in charge').
'a'+'tom'+'ic'='ATOMIC'
(Other definitions for atomic that I've seen before include "Extremely small" , "Based on nuclear energy" , "Very little" , "Relating to basic particles" , "Relating to tiny particles" .)