Function of powerful whip cracking (9)
I believe the answer is:
cotangent
'function' is the definition.
(cotangent is a kind of function)
'powerful whip cracking' is the wordplay.
'powerful' becomes 'cogent' (I've seen this before).
'whip' becomes 'tan' (I've seen this in another clue).
'cracking' indicates putting letters inside (inserted letters go into a gap or crack).
'cogent' going around 'tan' is 'COTANGENT'.
'of' is the link.
(Other definitions for cotangent that I've seen before include "Mathematical ratio" , "proportion" , "In triangular relationship" , "Maths function" , "Mathematician's function" .)