Head of big bolt for cart (6)
I believe the answer is:
barrow
'for cart' is the definition.
(barrow is a kind of cart)
'head of big bolt' is the wordplay.
'head of' indicates taking the first letters.
'bolt' becomes 'arrow' (I've seen this before).
The first letter of 'big' is 'b'.
'b'+'arrow'='BARROW'
(Other definitions for barrow that I've seen before include "where items are sold" , "Garden cart" , "Grave mound - wheeled handcart" , "Cart for the river?" , "Handcart; burial mound" .)