How hard I am on little Ronnie! (4)

I believe the answer is:

iron

I'm a little stuck... Click here to teach me more about this clue!

'how hard' is the definition.
'iron' can be an answer for 'hard' (thesaurus). I'm unsure of the 'how' bit.

'i am on little ronnie' is the wordplay.
'am' becomes 'on' (I can't justify this - if you can you should believe this answer much more).
'on' is a charade indicator (letters next to each other).
'little' suggests taking the first letters.
The first letter of 'ronnie' is 'r'.
'on' put after 'r' is 'ron'.
'i'+'ron'='IRON'

Can you help me to learn more?

(Other definitions for iron that I've seen before include "Main component of steel" , "Metallic element, symbol Fe" , "Hard metal" , "used in the evening" , "Metal (that may be wrought)" .)

I've seen this clue in the Evening Standard.

Want a hint initially instead of a full solution? Install my app