Finally, detailed map (5)
I believe the answer is:
atlas
'map' is the definition.
(thesaurus)
'finally detailed' is the wordplay.
'finally' becomes 'atlast' (synonyms).
'detailed' means to remove the last letter (the word must be end-less or 'de-tailed').
'atlast' with its last letter taken off is 'ATLAS'.
(Other definitions for atlas that I've seen before include "God" , "Album of maps" , "Supporter of the classics" , "Ballistic missile" , "Legendary weightlifter" .)