25 across
Stand-in
to
make
score
of
ten,
say
(6)
Stand-in to make score of ten, say (6)
hints
0
checks
0
hints
The definition is at the beginning of the clue.
How the clue works
The definition is Stand-in, since a DOUBLE is someone who stands in for another person (e.g. a stunt double). The rest of the clue works as a cryptic hint: to "make a score" (i.e. 20) out of ten, you'd need to DOUBLE it. "Say" signals that this is an example of the concept rather than a literal instruction. A neat clue that plays on the word "score" meaning twenty.