Front of legs, covered up by seaside, go white (6)
I believe the answer is:
bleach
'go white' is the definition.
(I've seen this before)
'front of legs covered up by seaside' is the wordplay.
'front of' says to take the initial letters (the letter at the front of the word).
'covered up by' is an insertion indicator.
'seaside' becomes 'beach' (a beach is found by the seaside).
The first letter of 'legs' is 'l'.
'l' going into 'beach' is 'BLEACH'.
(Other definitions for bleach that I've seen before include "Caustic material" , "Household cleanser" , "Clean" , "Make white or faded" , "Whitening chemical" .)