Having, at one point, a smash hit (4)
I believe the answer is:
with
'having' is the definition.
(I've seen this in another clue)
'at one point a smash hit' is the wordplay.
'at' means one lot of letters go next to another.
'one point' becomes 'w' (W is an example).
'a smash' is an anagram indicator.
'hit' with letters rearranged gives 'ith'.
'w'+'ith' is 'WITH'.
(Other definitions for with that I've seen before include "Accompanying" , "By means of" , "understanding" , "In spite of" , "In the company of" .)