When twenty-two hundred hours, put on loud tie (6)
I believe the answer is:
fasten
'tie' is the definition.
(I know that fasten is the is a more specific form of the action of tie.)
'when twenty-two hundred hours put on loud' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'twenty-two hundred hours' becomes 'ten' (10 pm on a twenty-four hour clock).
'put on' says to put letters next to each other (some letters go on or after others).
'loud' becomes 'f' (short for forte - 'loud' in music).
'as'+'ten'='asten'
'asten' after 'f' is 'FASTEN'.
(Other definitions for fasten that I've seen before include "Close, make secure" , "Affix" , "Tie or join together, fix firmly" , "Fix to, secure" , "Fix securely" .)