Character of flowing magma (5)
I believe the answer is:
gamma
'character' is the definition.
(gamma is a kind of letter of the alphabet)
'flowing magma' is the wordplay.
'flowing' is an anagram indicator.
'magma' is an anagram of 'GAMMA'.
'of' is the link.
(Other definitions for gamma that I've seen before include "Third letter of the Greek alphabet" , "Letter after alpha, beta" , "A bit of Greek" , "Greek character" , "third highest grade" .)