Painstakingly careful (8)
I believe the answer is:
thorough
'painstakingly careful' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for thorough that I've seen before include "Attentive to detail" , "Attending to every detail - complete" , "Scrupulous in completing work" , "Painstakingly careful and accurate" , "Assiduous, painstaking" .)