Cover up a case of these and expand (6)
I believe the answer is:
dilate
'expand' is the definition.
(thesaurus)
'cover up a case of these' is the wordplay.
'cover' becomes 'lid' (lid is a kind of cover**).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'case of' means to remove the middle letters (outsides of).
'these' with its centre removed is 'te'.
'lid' written backwards gives 'dil'.
'dil'+'a'+'te'='DILATE'
'and' is the link.
(Other definitions for dilate that I've seen before include "Swell" , "Expand in detail" , "Widen, spread out" , "Widen as eyes may" , "Enlarge - detail (anag)" .)