Undersized person consuming port to grow hugely (3,4)
I believe the answer is:
run riot
'grow hugely' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.
'undersized person consuming port' is the wordplay.
'undersized person' becomes 'runt' (I can't explain this - if you can you should give a lot more credence to this answer).
'consuming' indicates putting letters inside.
'port' becomes 'rio' (Rio de Janeiro is a port).
'runt' placed around 'rio' is 'RUN RIOT'.
'to' acts as a link.
Can you help me to learn more?
(Other definitions for run riot that I've seen before include "high-scoring test match" , "Behave unrestrainedly" , "Behave wildly and without restraint" , "Spread uncontrollably" , "Raise hell" .)