Head of exceptionally light African antelope (5)
I believe the answer is:
eland
'african antelope' is the definition.
(I've seen this before)
'head of exceptionally light' is the wordplay.
'head of' says to take the initial letters.
'light' becomes 'land' (I've seen this before**).
The initial letter of 'exceptionally' is 'e'.
'e'+'land'='ELAND'
(Other definitions for eland that I've seen before include "wild creature" , "One browses in the African plain" , "Antelope seen by Africans" , "Led an (anag) -- antelope" , "Large African deer with short spiral horns" .)