Peculiar heir is very likely to succeed (4-2)
I believe the answer is:
odds-on
'very likely to succeed' is the definition.
(I've seen this before)
'peculiar heir' is the wordplay.
'peculiar' becomes 'odd' (I've seen this in another clue).
'heir' becomes 'son' (I've seen this before).
'odd'+'son'='ODDS-ON'
'is' acts as a link.
(Other definitions for odds-on that I've seen before include "With success more likely than favour in bookie's phrase" , "With success more likely as bookie would calculate" , "Very likely to happen" , "With success more likely than failure - check with bookie" , "More likely to win than lose" .)