Hoskins is who I am! One ghastly twerp essentially without limits! (7)
I believe the answer is:
immense
'hoskins is' is the definition.
I can't tell whether this defines the answer.
'i am one ghastly twerp essentially without limits' is the wordplay.
'i am' becomes 'im'.
'one' becomes 'me' (a setter might use 'one' to humourously mean 'me').
'ghastly twerp' becomes 'ns' (I can't explain this - if you can you should give a lot more credence to this answer).
'essentially' becomes 'atheart' (I've seen this before).
'without limits' means to look at the middle letters (letters at the edges or 'limits' are removed).
The central letter of 'atheart' is 'e'.
'im'+'me'+'ns'+'e'='IMMENSE'
'who' acts as a link.
Can you help me to learn more?
(Other definitions for immense that I've seen before include "Unusually great in size" , "'Gigantic, extensive (7)'" , "Huge, vast" , "Unctuously ingratiating" , "Vast or enormous" .)