Goal of peer is to join queen (9)
I believe the answer is:
equaliser
'goal' is the definition.
(I've seen this before)
'peer is to join queen' is the wordplay.
'peer' becomes 'equal' ('equal' can be a synonym of 'peer'**).
'to join' means one lot of letters go next to another.
'queen' becomes 'er' (abbreviation for Elizabeth Regina).
'equal'+'is'+'er'='EQUALISER'
'of' is the link.
(Other definitions for equaliser that I've seen before include "One may demand extra time" , "weapon, perhaps" , "reply that might make one point" , "Levelling goal" , "Goal to tie" .)