Rambling sir blethers on, manifesting fraternal spirit (13)
I believe the answer is:
brotherliness
'fraternal spirit' is the definition.
'rambling sir blethers on' is the wordplay.
'rambling' is an anagram indicator.
'sir'+'blethers'+'on'='sirbletherson'
'sirbletherson' is an anagram of 'BROTHERLINESS'.
'manifesting' is the link.
(Other definitions for brotherliness that I've seen before include "boys' friendly relationship" , "amicable relations" .)