On fishing for orca, I spy tuna's snout — that's fishy (9)

'on fishing' is the definition.
(piscatory means relating to fishing)

'orca i spy tuna's snout that's fishy' is the wordplay.
'snout' suggests taking the first letters (the front of the word).
'that's fishy' indicates anagramming the letters (a strange or fishy spelling).
The initial letter of 'tunas' is 't'.
'orca'+'i'+'spy'+'t'='orcaispyt'
'orcaispyt' is an anagram of 'PISCATORY'.

'for' is the link.