Stop to take in every second of big blue fish (7)
I believe the answer is:
halibut
'fish' is the definition.
(halibut is a kind of fish)
'stop to take in every second of big blue' is the wordplay.
'stop' becomes 'halt' ('halt' can be a synonym of 'stop').
'to take in' indicates putting letters inside.
'every second of' indicates alternate letters.
'big'+'blue'='bigblue'
The alternating letters of 'bigblue' are 'ibu'.
'halt' placed around 'ibu' is 'HALIBUT'.
(Other definitions for halibut that I've seen before include "Something fishy here" , "Utah lib (anag)" , "One's scaled" , "Fishy sort" , "Marine food fish" .)