Keep an eye on porter gathering fish but wasting hour (5,3)
I believe the answer is:
stake out
'gathering fish but wasting hour' is the definition.
I don't know anything about this answer so I cannot judge whether this works.
'keep an eye on porter' is the wordplay.
'keep an eye' becomes 'ake' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'on' is an insertion indicator.
'porter' becomes 'stout' (synonyms).
'ake' placed inside 'stout' is 'STAKE-OUT'.
Can you help me to learn more?
(Other definitions for stake out that I've seen before include "Observe" , "police surveillance" , "Place under surveillance" , "Surveillance mission" .)