Feeble month in regard to colliery (8)
I believe the answer is:
decrepit
'feeble' is the definition.
('decrepit' can be a synonym of 'feeble')
'month in regard to colliery' is the wordplay.
'month' becomes 'dec' (Dec is an example).
'in regard' becomes 're' (I can't justify this - if you can you should believe this answer much more).
'to colliery' becomes 'pit' ('pit' can be a synonym of 'colliery').
'dec'+'re'+'pit'='DECREPIT'
Can you help me to learn more?
(Other definitions for decrepit that I've seen before include "Worn and broken down by use" , "Doddering" , "Run-down by old age or hard use" , "Worn out; infirm" , "In the last stages of decay" .)