Ensigns following trails (5)
I believe the answer is:
flags
'ensigns' is the definition.
('flag' can be a synonym of 'ensign')
'following trails' is the wordplay.
'following' becomes 'f' (used when citing page numbers etc).
'trails' becomes 'lags' (I've seen this in another clue).
'f'+'lags'='FLAGS'
(Other definitions for flags that I've seen before include "Standards" , "Droops" , "Grows languid" , "Tires; national symbols" , "Declines" .)