Second double fifty secured by man (9)
I believe the answer is:
twinkling
'second' is the definition.
(I've seen this before)
'double fifty secured by man' is the wordplay.
'double' becomes 'twin' (I've seen this before).
'fifty' becomes 'l' ('L' can be a synonym of 'fifty').
'secured by' means one lot of letters goes inside another (I've seen this in other clues).
'man' becomes 'king' (king is a kind of man**).
'l' placed inside 'king' is 'kling'.
'twin'+'kling'='TWINKLING'
(Other definitions for twinkling that I've seen before include "for a short time" , "Sparkling like a star" , "How stars appear" , "Instant" , "Second" .)