Boy has caught one to become heir (5)
I believe the answer is:
scion
'heir' is the definition.
(I've seen this before)
'boy has caught one' is the wordplay.
'boy' becomes 'son' (synonyms).
'has' indicates putting letters inside.
'caught' becomes 'c' (cricket abbreviation).
'one' becomes 'i' (Roman numeral).
'c'+'i'='ci'
'son' placed around 'ci' is 'SCION'.
'to become' is the link.
(Other definitions for scion that I've seen before include "Offshoot" , "Young family member" , "Young shoot/twig" , "Young shoot or twig" , "Coins (anag)" .)