Uniting people on outside of centre at the sound of the bell (9)

'uniting' is the definition.
Although both the answer and definition are present participle verbs, I can't see how they can define each other.

'people on outside of centre at the sound of the bell' is the wordplay.
'people' becomes 'men' (men are people).
'on' says to put letters next to each other.
'outside of' suggests removing the centre.
'at' is a charade indicator (letters next to each other).
'the sound of the bell' becomes 'ting' (I can't justify this - if you can you should give a lot more credence to this answer).
'centre' with its centre taken out is 'ce'.
'men' put after 'ce' is 'cemen'.
'cemen'+'ting'='CEMENTING'

Can you help me to learn more?