Surprisingly help a mere transient (9)
I believe the answer is:
ephemeral
'transient' is the definition.
('ephemeral' can be a synonym of 'transient')
'surprisingly help a mere' is the wordplay.
'surprisingly' is an anagram indicator.
'help'+'a'+'mere'='helpamere'
'helpamere' with letters rearranged gives 'EPHEMERAL'.
(Other definitions for ephemeral that I've seen before include "staying for a little while" , "Short-lived, passing quickly" , "Short lived" , "just for a day?" , "Having only brief existence" .)