Elaborate, but formerly the opposite, apparently (7)
I believe the answer is:
explain
'elaborate' is the definition.
(thesaurus)
'formerly the opposite apparently' is the wordplay.
'formerly' becomes 'ex'.
'the opposite apparently' becomes 'plain' (I am not sure about this - if you are sure you should believe this answer much more).
'ex'+'plain'='EXPLAIN'
'but' acts as a link.
Can you help me to learn more?
(Other definitions for explain that I've seen before include "Describe" , "Elucidate - define" , "Shed light" , "Make understandable" , "Resolve" .)