Mostly try to score before Hearts in tie (6)
I believe the answer is:
attach
'tie' is the definition.
(to tie is to link or attach)
'mostly try to score before hearts' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'try to score' becomes 'attack' (eg attack in football).
'before' says to put letters next to each other.
'hearts' becomes 'h' (abbreviation used in card games e.g. bridge).
'attack' with its last letter taken away is 'attac'.
'attac'+'h'='ATTACH'
'in' is the link.
(Other definitions for attach that I've seen before include "Affix, connect" , "Join or seize by law" , "Annex" , "Glue" , "Append, affix" .)