Past agreement with heartless Terence needs attention (10)
I believe the answer is:
yesteryear
'past' is the definition.
(synonyms)
'agreement with heartless terence needs attention' is the wordplay.
'agreement with' becomes 'yes' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'heartless' suggests removing the centre.
'terence' becomes 'Terry' (Terry is short for Terence).
'needs' says to put letters next to each other.
'attention' becomes 'ear' (having someone's ear is having their attention).
'terry' with its centre taken out is 'tery'.
'yes'+'tery'+'ear'='YESTERYEAR'
Can you help me to learn more?
(Other definitions for yesteryear that I've seen before include "Days gone by" , "Twelve months ago" , "Days of old (poet.)" , "nostalgic time, perhaps" , "Period 14 [CENTENARIAN] lived through" .)