Clue: Then slowly rue friend welcoming Harry's clue? (5,2,5)
Answer: MARRY IN HASTE
Explanation: referring to the saying 'Marry in haste, repent at leisure' with 'repent at leisure'="slowly rue"
 
MATE="friend" around RRY IN HAS
 
as part of a clue's instructions, putting RRY IN HAS could give HA-RRY-S i.e. "Harry's"