Determined boy conceals alien (3,2)
I believe the answer is:
set on
'determined' is the definition.
(I've seen this before)
'boy conceals alien' is the wordplay.
'boy' becomes 'son' (male child).
'conceals' means one lot of letters goes inside another.
'alien' becomes 'et' (extra terrestrial).
'son' enclosing 'et' is 'SET ON'.
(Other definitions for set on that I've seen before include "Go" , "Determined" , "Attack" .)