Chief investigator ultimately cracking handwriting (5)
I believe the answer is:
first
'chief' is the definition.
(I've seen this in another clue)
'investigator ultimately cracking handwriting' is the wordplay.
'ultimately' indicates one should take the final letters.
'cracking' is an insertion indicator (inserted letters go into a gap or crack).
'handwriting' becomes 'fist' (I've seen this before).
The last letter of 'investigator' is 'r'.
'r' placed within 'fist' is 'FIRST'.
(Other definitions for first that I've seen before include "second to none" , "In winning position" , "higher than the others" , "Original" , "Pre-eminent" .)