Hit hard in stomach with power (5)
I believe the answer is:
thump
'hit' is the definition.
(I know that thump is a more specific form of the action hit)
'hard in stomach with power' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'in' means one lot of letters goes inside another.
'stomach' becomes 'tum' (synonyms).
'with' is a charade indicator (letters next to each other).
'power' becomes 'p' (symbol in physics).
'h' inserted into 'tum' is 'thum'.
'thum'+'p'='THUMP'
(Other definitions for thump that I've seen before include "Punch" , "clobber" , "Hit forcefully" , "Pound" , "Hit - defeat heavily" .)