Excuse is old and mostly hollow (6)
I believe the answer is:
exempt
'excuse' is the definition.
(synonyms)
'old and mostly hollow' is the wordplay.
'old' becomes 'ex'.
'and' means one lot of letters go next to another.
'mostly hollow' becomes 'empt' (I can't explain this - if you can you should believe this answer much more).
'ex'+'empt'='EXEMPT'
'is' is the link.
Can you help me to learn more?
(Other definitions for exempt that I've seen before include "Not subject to, taxation perhaps" , "not expected to pay" , "Freed from liability (to tax etc)" , "Not subject to or liable for" , "Free (from obligation)" .)