Summer treat best to keep cold (4-3)
I believe the answer is:
choc-ice
'summer treat' is the definition.
'best to keep cold' is the wordplay.
'best' becomes 'choice' (I've seen this before**).
'to keep' means one lot of letters goes inside another.
'cold' becomes 'c' (eg on taps).
'choice' placed around 'c' is 'CHOC-ICE'.
(Other definitions for choc-ice that I've seen before include "Frozen sweet" , "Cold treat" , "Cold, coated confection" , "Frozen confection" , "Frozen bar" .)