The most common and most likely answer to this crossword clue is the 6 plus 7 letter word SUMMER PUDDING
I've seen this crossword clue in Financial Times
S
U
M
M
E
R
P
U
D
D
I
N
G
More clues leading to the result SUMMER PUDDING
Buddy explains!
The straight part
We know that "Winter starter? On the contrary" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case winter starter? on the contrary = summer pudding.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.