Formidable number circling sides of tower (6)
I believe the answer is:
strong
'formidable' is the definition.
(thesaurus)
'number circling sides of tower' is the wordplay.
'number' becomes 'song' (I've seen this before**).
'circling' indicates putting letters inside (circling can mean going around or containing).
'sides of' means to remove the middle letters (only the letters on the sides remain).
'tower' with its middle removed is 'tr'.
'song' enclosing 'tr' is 'STRONG'.
(Other definitions for strong that I've seen before include "Mighty, powerful" , "is excelling" , "Powerful; full-flavoured" , "Solid" , "Great in power" .)