Following everything son declines (5)
I believe the answer is:
falls
'declines' is the definition.
(I know this)
'following everything son' is the wordplay.
'following' becomes 'f' (used when citing page numbers etc).
'everything' becomes 'all' (all things).
'son' becomes 's' (genealogical abbreviation for son).
'f'+'all'+'s'='FALLS'
(Other definitions for falls that I've seen before include "North Country force" , "Forces" , "Drops" , "Goes down" , "suffers defeat" .)