Fish: Henry has one only (7)
I believe the answer is:
halibut
'fish' is the definition.
(halibut is a kind of fish)
'henry has one only' is the wordplay.
'henry' becomes 'hal'.
'has' says to put letters next to each other.
'one' becomes 'i' (Roman numeral).
'only' becomes 'but' (I've seen this before).
'hal'+'i'+'but'='HALIBUT'
(Other definitions for halibut that I've seen before include "Swimmer" , "Utah lib (anag)" , "Marine food fish" , "Large edible flatfish" , "Large food fish" .)