Grounds, say, to go after head of Exxon (6)
I believe the answer is:
estate
'grounds' is the definition.
(I've seen this before)
'say to go after head of exxon' is the wordplay.
'say' becomes 'state' (synonyms).
'to go after' means one lot of letters go next to another.
'head of' suggests taking the first letters.
The initial letter of 'exxon' is 'e'.
'state' after 'e' is 'ESTATE'.
(Other definitions for estate that I've seen before include "Rank - housing development - car" , "All of one's assets after death" , "A group of buildings built together in one development" , "(see 1)" , "All that one owns at death" .)