Assiduous old man in street meeting a top man (11)
I believe the answer is:
painstaking
'assiduous' is the definition.
(thesaurus)
'old man in street meeting a top man' is the subsidiary indication.
'old man' becomes 'pa' ('pa' can be a synonym of 'old man'**).
'street' becomes 'st'.
'meeting' says to put letters next to each other.
'top man' becomes 'king'.
'pa'+'in'+'st'+'a'+'king'='painstaking'
(Other definitions for painstaking that I've seen before include "using great care" , "Laboriously careful" , "Meticulous" , "Extremely careful" , "Assiduous, thorough" .)