A welcome surprise as dogs run to the finish (7)
I believe the answer is:
godsend
'a welcome surprise' is the definition.
(I've seen this before)
'dogs run to the finish' is the wordplay.
'run' is an anagram indicator.
'to the finish' becomes 'end' (I've seen this in another clue).
'dogs' with letters rearranged gives 'gods'.
'gods'+'end'='GODSEND'
'as' acts as a link.
(Other definitions for godsend that I've seen before include "The relief" , "Lucky find" , "Welcome good fortune" , "Something unexpected but welcome" , "Welcome surprise" .)