Fabulous beasts with one ear in sun, gambolling (8)
I believe the answer is:
unicorns
'fabulous beasts' is the definition.
(fabulous can mean mythical)
'one ear in sun gambolling' is the wordplay.
'one' becomes 'I' (Roman numeral).
'ear' becomes 'corn' (as in an ear of corn).
'in' is an insertion indicator.
'gambolling' is an anagram indicator (letters run or gambol around).
'i'+'corn'='icorn'
'sun' is an anagram of 'uns'.
'icorn' placed within 'uns' is 'UNICORNS'.
'with' acts as a link.
(Other definitions for unicorns that I've seen before include "Mythical horses with horns on foreheads" , "Coins run off these imaginary creatures" , "Mythical animals like single-horned horses" , "Mythical creatures like single-horned horses" , "Imaginary one-horned animals" .)