A right pain. Unfinished nests of arthropods (9)

'arthropods' is the definition.
(arachnid is a kind of arthropod)

'a right pain unfinished nests' is the wordplay.
'right' becomes 'r' (common abbreviation).
'pain' becomes 'ache' (ache is a kind of pain).
'unfinished' means to remove the last letter.
'nests' becomes 'nids' (I can't justify this - if you can you should give a lot more credence to this answer).
'ache' with its last letter taken off is 'ach'.
'a'+'r'+'ach'+'nids'='ARACHNIDS'

'of' acts as a link.

Can you help me to learn more?