A pole has not curved (5)
I believe the answer is:
round
'curved' is the definition.
(thesaurus)
'a pole has not' is the wordplay.
'a pole' becomes 'rod' ('rod' can be a synonym of 'pole').
'has' indicates putting letters inside.
'not' becomes 'un' (prefix meaning 'not').
'rod' going around 'un' is 'ROUND'.
(Other definitions for round that I've seen before include "Ignore fractions" , "18 holes of golf" , "Like ball < in rough" , "A partsong in which voices follow each other" , "Shaped like a circle" .)