Stop to take in every second of big blue fish (7)
I believe the answer is:
Here is my best explanation:
'fish' is the definition.
(halibut is a kind of fish)
'stop to take in every second of big blue' is the subsidiary indication.
'stop' becomes 'halt' ('halt' can be a synonym of 'stop').
'to take in' means one lot of letters goes inside another.
'every second of' means one should take alternating letters.
The alternate letters of 'bigblue' are 'ibu'.
'halt' placed around 'ibu' is 'halibut'.
(Other definitions for halibut that I've seen before include "Largest flatfish", "Large edible flatfish", "Marine food fish", "Swimmer", "Fish found in Bali hut", "Food fish", "Utah lib (anag)", "Something fishy here", "Flatfish" and "One's scaled".)