Relative trapping animal in bottle (6)
I believe the answer is:
magnum
'bottle' is the definition.
(magnum is a kind of bottle)
'relative trapping animal' is the wordplay.
'relative' becomes 'mam' (informal term for mother).
'trapping' indicates putting letters inside.
'animal' becomes 'gnu' (type of antelope).
'mam' enclosing 'gnu' is 'MAGNUM'.
'in' acts as a link.
(Other definitions for magnum that I've seen before include "Large bottle of champagne, twice normal size" , "1 1/2 litre champagne bottle" , "Double-size wine bottle" , "a lot of bottle!" , "Just one bottle of champagne?" .)