They change locks in doors once you blocked holes? (5)
I believe the answer is:
dyers
'they change locks in doors' is the definition.
The definition suggests an adverb but the answer is not.
'you blocked holes?' is the wordplay.
'you' becomes 'yer' ('yer' is a dialect form of 'you').
'blocked' indicates putting letters inside (to block can mean to box in or contain).
'holes?' becomes 'ds' (I can't explain this - if you can you should give a lot more credence to this answer).
'yer' inserted inside 'ds' is 'DYERS'.
'once' acts as a link.
Can you help me to learn more?
(Other definitions for dyers that I've seen before include "Stainers of cloth" , "Colour-changers" , "14 [SWANUPPING] company" .)