Grasp lead item in constitutional legislation (4)
I believe the answer is:
claw
'grasp' is the definition.
(to grasp with a claw)
'lead item in constitutional legislation' is the wordplay.
'lead item in' indicates taking the first letters (the letter which leads the word).
'legislation' becomes 'law' (I've seen this in other clues).
The initial letter of 'constitutional' is 'c'.
'c'+'law'='CLAW'
(Other definitions for claw that I've seen before include "Rake" , "Pointed nail on animal's foot" , "See 24" , "Hook on the end of toe" , "Lobster's grasping structure" .)