Head off havoc and fuss with an alligator pear (7)
I believe the answer is:
avocado
'an alligator pear' is the definition.
('avocado' can be a synonym of 'alligator pear')
'head off havoc and fuss' is the wordplay.
'head off' means to remove the first letter.
'and' says to put letters next to each other.
'fuss' becomes 'ado' (synonyms).
'havoc' with its first letter removed is 'avoc'.
'avoc'+'ado'='AVOCADO'
'with' acts as a link.
(Other definitions for avocado that I've seen before include "A dark green pear with cramy flesh" , "Fruit with large stone" , "Blackish- or yellowish-green colour" , "A fruit; a dull green" , "Tropical tree - edible green-skinned fruit" .)