Tie up and hit hard (4)
I believe the answer is:
tonk
'hit hard' is the definition.
(I've seen this before)
'tie up' is the wordplay.
'tie' becomes 'knot' (knotting is a kind of tiing).
'up' is a reversal indicator (in down clue: letters go upwards).
'knot' backwards is 'TONK'.
'and' acts as a link.
(Other definitions for tonk that I've seen before include "Rod" , "from Australia is hit" , "Hit hard" .)