One in a tub, or one in a crate? (7)
I believe the answer is:
aviator
'one in a crate?' is the definition.
('crate' is an old-fashioned slang term for plane)
'one in a tub or' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' means one lot of letters goes inside another.
'tub' becomes 'vat' ('vat' can be a synonym of 'tub').
'a'+'vat'='avat'
'i' put into 'avat' is 'aviat'.
'aviat'+'or'='AVIATOR'
(Other definitions for aviator that I've seen before include "Airman" , "Ace, say," , "Flyer in early days of planes" , "Air pilot" , "one in a crate?" .)