Inquisitive navigators originally discovered cetaceans respire (6)
I believe the answer is:
inhale
'respire' is the definition.
(inhaling is a kind of respiring)
'inquisitive navigators originally discovered cetaceans' is the wordplay.
'originally' says to take the initial letters.
'discovered' says to take the centre (dis-covered - outer letters removed).
'cetaceans' becomes 'whales' (whale is a kind of cetacean).
The middle letters of 'whales' are 'hale'.
The initial letters of 'inquisitive navigators' is 'in'.
'in'+'hale'='INHALE'
(Other definitions for inhale that I've seen before include "drag" , "Draw air into lungs" , "Whiff" , "Breathe in (gas, smoke)" , "Draw breath" .)