To change once drilled is badly thought out (3-10)
I believe the answer is:
ill-considered
'badly thought out' is the definition.
(I've seen this before)
'to change once drilled is' is the wordplay.
'to change' indicates an anagram.
'once'+'drilled'+'is'='oncedrilledis'
'oncedrilledis' with letters rearranged gives 'ILL-CONSIDERED'.
(Other definitions for ill-considered that I've seen before include "Foolish" , "End collier I/Ds? (anag)" , "Rash" , "Badly thought out" , "Not carefully planned" .)