A second son for the prophet (4)
I believe the answer is:
amos
'the prophet' is the definition.
(I've seen this before)
'a second son' is the wordplay.
'second' becomes 'mo' (both are short periods of time).
'son' becomes 's'.
'a'+'mo'+'s'='AMOS'
'for' acts as a link.
(Other definitions for amos that I've seen before include "30th OT book" , "Prophet in the Bible" , "Hebrew prophet" , "Biblical character" , "man" .)