Thinner race that's in northern region initially (7)
I believe the answer is:
runnier
'thinner' is the definition.
Both the answer and definition are comparative adjectives. Perhaps you can see an association between them that I don't see?
'race that's in northern region initially' is the wordplay.
'race' becomes 'run' (synonyms).
'that's' becomes 'ie' (abbreviation for 'id est', 'that is' in Latin).
'in' is an insertion indicator.
'initially' suggests taking the first letters.
The first letters of 'northern region' is 'nr'.
'ie' inserted within 'nr' is 'nier'.
'run'+'nier'='RUNNIER'
Can you help me to learn more?
(Other definitions for runnier that I've seen before include "More liquid" , "becomes more fluid ..." .)