Place sandwiches on starter of cucumber chopped in advance (3-3)

'chopped in advance' is the definition.
(cut in advance)

'place sandwiches on starter of cucumber' is the wordplay.
'place' becomes 'put' (synonyms).
'sandwiches' means one lot of letters goes inside another (to sandwich is to block something in).
'on' becomes 're' ('on' can be mean 'with reference to').
'starter of' indicates taking the first letters.
The first letter of 'cucumber' is 'c'.
're'+'c'='rec'
'put' enclosing 'rec' is 'PRE-CUT'.