Hit seen at No.1 among the hits which are considered key (6)
I believe the answer is:
thwack
'hit' is the definition.
(I know that thwack is a type of hit)
'seen at no 1 among the hits which are considered key' is the wordplay.
'seen at no 1 among' says to take the initial letters (letters in 1st position).
The initial letters of 'the hits which are considered key' is 'THWACK'.
(Other definitions for thwack that I've seen before include "Strike forcefully" , "Hard blow with a flat object" , "Hit with something flat" , "Wallop" , "Beat with something flat" .)