Father's boy has one temper (7)
I believe the answer is:
passion
'temper' is the definition.
(thesaurus)
'father's boy has one' is the wordplay.
'father's' becomes 'pas' (pa is a kind of father).
'boy' becomes 'son' ('son' can be a synonym of 'boy'**).
'has' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'son' placed around 'i' is 'sion'.
'pas'+'sion'='PASSION'
(Other definitions for passion that I've seen before include "Intense enthusiasm for something" , "Intense emotion, anger or love" , "Ardour, fervour" , "Strong emotion, anger or love" , "Strong feeling, or anger or desire possibly" .)