A Gallic disposition is exceptionally cold (7)
I believe the answer is:
glacial
'exceptionally cold' is the definition.
'glacial' can be an answer for 'cold' (similar in meaning). I'm not certain of the 'exceptionally' bit.
'a gallic disposition' is the wordplay.
'disposition' indicates anagramming the letters.
'a'+'gallic'='agallic'
'agallic' with letters rearranged gives 'GLACIAL'.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for glacial that I've seen before include "Icy; distant" , "Relating to ice" , "Pertaining to the ice ages" , "Ice cold" , "Cold and icy" .)