Visitor all but seconds judge (5)
I believe the answer is:
guess
'judge' is the definition.
(I know that judge can be written as guess)
'visitor all but seconds' is the wordplay.
'visitor' becomes 'guest' (someone visiting a place).
'all but' means to remove the last letter ('all but' can mean 'almost' or 'nearly').
'seconds' becomes 's'.
'guest' with its final letter taken away is 'gues'.
'gues'+'s'='GUESS'
(Other definitions for guess that I've seen before include "Speculation" , "Take a stab at the answer" , "Dare say" , "Speculate" , "Estimate on the basis of little information" .)