Having covered just the beginning of easy subject, go to bed with an unresolved grievance (4,2,4)

'an unresolved grievance' is the definition.

'having covered just the beginning of easy subject go to bed' is the wordplay.
'having covered' means one lot of letters goes inside another.
'just the beginning of' suggests taking the first letters.
'subject' becomes 'topic' ('topic' can be a synonym of 'subject'**).
'go to bed' becomes 'bonk'.
The initial letter of 'easy' is 'e'.
'e'+'topic'='etopic'
'etopic' put into 'bonk' is 'BONE TO PICK'.

'with' is the link.