Should be “Anything to declare?” (5)
I believe the answer is:
ought
'should be' is the definition.
(I've seen this before)
'anything to declare?' is the wordplay.
'anything' becomes 'aught' ('aught' means anything).
'to declare?' indicates a 'sounds like' (homophone) clue (I've seen 'declared' mean this).
'aught' is a homophone of 'OUGHT'.
(Other definitions for ought that I've seen before include "outdated anything at all?" , "Be bound" , "Be logically necessary" , "Tough (anag)" , "Strange thing" .)