Son took meal to fill right up (4)
I believe the answer is:
sate
'fill right up' is the definition.
(to sate oneself is to gorge on food)
'son took meal' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'took meal' becomes 'ate' (ate a meal).
's'+'ate'='SATE'
'to' acts as a link.
(Other definitions for sate that I've seen before include "Satisfy completely" , "replete" , "Make content" , "cloy" , "Quench" .)