Comprehensive, even if approximate (8)
I believe the answer is:
thorough
'comprehensive' is the definition.
(similar in meaning)
'even if approximate' is the wordplay.
'even if' becomes 'tho'' (I've seen this in other clues).
'approximate' becomes 'rough' (I've seen this before).
'tho'+'rough'='THOROUGH'
(Other definitions for thorough that I've seen before include "Doing things with great attention to detail" , "Not superficial or partial" , "Complete - painstaking - unqualified" , "Complete; utter" , "Attending to every detail - complete" .)