Father's heir has one temper (7)
I believe the answer is:
passion
'temper' is the definition.
(thesaurus)
'father's heir has one' is the wordplay.
'father's' becomes 'pas' (pa is a kind of father).
'heir' becomes 'son' (I've seen this before).
'has' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'son' placed around 'i' is 'sion'.
'pas'+'sion'='PASSION'
(Other definitions for passion that I've seen before include "Strong feeling of intense emotion" , "Rage" , "Ardour, fervour" , "thing one really cares about" , "Intense emotion, anger or love" .)