Awkward to handle cucumber, so messy sandwiches (10)
I believe the answer is:
cumbersome
'awkward' is the definition.
(I know that awkward can be written as cumbersome)
'handle cucumber so messy sandwiches' is the wordplay.
'sandwiches' indicates the answer is hidden within the clue (the answer is sandwiched in by the clue).
'CUMBERSOME' can be found hidden inside 'handle cucumber so messy'.
'to' is the link.
(Other definitions for cumbersome that I've seen before include "Awkward" , "Unwieldy" .)