Polish end of button in blue (4)
I believe the answer is:
sand
'polish' is the definition.
(I've seen this before)
'end of button in blue' is the wordplay.
'end of' suggests the final letters.
'in' means one lot of letters goes inside another.
'blue' becomes 'sad' (similar in meaning).
The final letter of 'button' is 'n'.
'n' inserted into 'sad' is 'SAND'.
(Other definitions for sand that I've seen before include "yellowish-brown" , "Sort of paper, piper, castle or martin" , "It's used to make glass" , "Fine grit" , "Very fine grains of rock, as on beach" .)