Rights to catch fish added as an additional clause (5)
'an additional clause' is the definition.
(additional clause on a contract or agreement)
'rights to catch fish' is the wordplay.
'rights' becomes 'rr'.
'to catch' is an insertion indicator.
'fish' becomes 'ide' (I've seen this before).
'rr' enclosing 'ide' is 'RIDER'.
'added as' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for rider that I've seen before include "Horseman or add-on verdict by jury", "given little weight", "one making journey", "Horseman or supplementary jury verdict", "Contractual clause".)