Old and grand wearing tie in concert (8)
I believe the answer is:
together
'in concert' is the definition.
(synonyms)
'old and grand wearing tie' is the wordplay.
'old' becomes 'o' (abbreviation).
'and' says to put letters next to each other.
'grand' becomes 'g' (G is short for grand as in a thousand).
'wearing' means one lot of letters goes inside another.
'tie' becomes 'tether' (I've seen this before).
'o'+'g'='og'
'og' inserted within 'tether' is 'TOGETHER'.
(Other definitions for together that I've seen before include "In conjunction, combined" , "In combination" , "In one another's company" , "as a group" , "In company or in conjunction" .)