Son has to study hard to beat it? (5)
I believe the answer is:
scram
'beat it?' is the definition.
(I've seen this before)
'son has to study hard' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'has' is a charade indicator (letters next to each other).
'to study hard' becomes 'cram' (to cram is to study hard or intensely).
's'+'cram'='SCRAM'
'to' is the link.
(Other definitions for scram that I've seen before include "Scarper" , "make emergency shut-down" , "Slangy order to go away, beat it" , "Go away quickly" , "Buzz off!" .)