United entering series, surprisingly put out again (7)
I believe the answer is:
reissue
'put out again' is the definition.
(to publish or put out again)
'united entering series surprisingly' is the wordplay.
'united' becomes 'U' (abbreviation).
'entering' means one lot of letters goes inside another.
'surprisingly' indicates an anagram (a surprising ordering of the letters).
'series' anagrammed gives 'reisse'.
'u' put inside 'reisse' is 'REISSUE'.
(Other definitions for reissue that I've seen before include "A second impression, perhaps" , "Republish" , "Again 22 across" , "New edition of book" , "Publication reprint" .)