Boy is right to keep up (4)
I believe the answer is:
neil
'boy' is the definition.
(I've seen this before)
'right to keep up' is the wordplay.
'right to keep' becomes 'lien' (legal right to keep property against a debt).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'lien' back-to-front is 'NEIL'.
'is' is the link.
(Other definitions for neil that I've seen before include "Boy's name" , "1969 moon walker" , "- Armstrong, first man to walk on the moon" , "-- Sedaka, US singer" , "-- Armstrong, Apollo astronaut" .)