Full, but approximate (8)
I believe the answer is:
thorough
'full' is the definition.
(similar in meaning)
'but approximate' is the wordplay.
'but' becomes 'tho' (I've seen this in other clues).
'approximate' becomes 'rough' (I've seen this in another clue).
'tho'+'rough'='THOROUGH'
(Other definitions for thorough that I've seen before include "Paying attention to detail, meticulous" , "Doing things with great attention to detail" , "'Exhaustive, complete (8)'" , "Complete, absolute" , "Passage" .)