Huge gap in the back of some waterproof luggage (4)
I believe the answer is:
gulf
'huge gap' is the definition.
'gulf' can be an answer for 'gap' (gulf is a kind of gap). I'm not sure about the 'huge' bit.
'back of some waterproof luggage' is the wordplay.
'back of' says the letters should be written backwards (I've seen 'back' mean this).
'some' indicates a hidden word (some of the parts of the word are used).
'flug' is hidden within 'waterproof luggage'.
'flug' reversed gives 'GULF'.
'in the' acts as a link.
Can you help me to learn more?
(Other definitions for gulf that I've seen before include "Wide gap; arm of sea" , "Unbridgeable disparity" , "Deep inlet of sea; chasm" , "Large deep bay" , "Wide separation" .)