One gaining ground learnt not to be impatient (10)
I believe the answer is:
intolerant
'impatient' is the definition.
(thesaurus)
'one gaining ground learnt not' is the wordplay.
'one' becomes 'i' (Roman numeral).
'gaining' is a charade indicator (letters next to each other).
'ground' indicates anagramming the letters (grind the letters ).
'learnt'+'not'='learntnot'
'learntnot' with letters rearranged gives 'ntolerant'.
'i'+'ntolerant'='INTOLERANT'
'to be' is the link.
(Other definitions for intolerant that I've seen before include "Lacking respect for the beliefs of others" , "Hating" , "Bigoted" , "Impatient" , "illiberal" .)