Realise a temperature ain't problematic (6)
I believe the answer is:
attain
'realise' is the definition.
(thesaurus)
'a temperature ain't problematic' is the wordplay.
'temperature' becomes 't' (abbreviation).
'problematic' indicates an anagram.
'aint' anagrammed gives 'tain'.
'a'+'t'+'tain'='ATTAIN'
(Other definitions for attain that I've seen before include "Reach (a specified level)" , "Succeed in doing, get there" , "Reach (age, size etc.)" , "Reach, get there as with ambition" , "Succeed in reaching, achieve" .)