Verify quietly on ramble (5)
I believe the answer is:
prove
'verify' is the definition.
(thesaurus)
'quietly on ramble' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'on' says to put letters next to each other.
'ramble' becomes 'rove' ('rove' can be a synonym of 'ramble').
'p'+'rove'='PROVE'
(Other definitions for prove that I've seen before include "Ascertain" , "Demonstrate beyond doubt" , "Demonstrate by evidence" , "Demonstrate conclusively" , "Establish the validity of" .)