Be quiet as English lieutenant enters bar backwards (4,2)
I believe the answer is:
belt up
'be quiet' is the definition.
(both can be used to tell someone to stop talking)
'english lieutenant enters bar backwards' is the wordplay.
'english' becomes 'E' (abbreviation as in OED).
'lieutenant' becomes 'lt' (abbreviation for lieutenant).
'enters' indicates putting letters inside.
'bar' becomes 'pub' (I've seen this before).
'backwards' says the letters should be written backwards.
'pub' back-to-front is 'bup'.
'e'+'lt'='elt'
'elt' put into 'bup' is 'BELT UP'.
'as' acts as a link.
(Other definitions for belt up that I've seen before include "To secure safety strap" , "Be quiet - as motorists should do" , "call for passenger safety" , "Strap yourself in - keep quiet!" , "Be quiet, or secure yourself in car seat" .)