Gives in regarding fast son (7)
I believe the answer is:
relents
'gives in' is the definition.
(I've seen this in another clue)
'regarding fast son' is the wordplay.
'regarding' becomes 're' ('re' means regarding or about).
'fast' becomes 'lent' (Lent is a period of fasting in Christianity).
'son' becomes 's' (genealogical abbreviation for son).
're'+'lent'+'s'='RELENTS'
(Other definitions for relents that I've seen before include "Comes round" , "Gives way" , "Abates, softens" , "Becomes less severe and more lenient" , "Eases off" .)