Popular old means of decreasing, not right for an increase (9)
I believe the answer is:
inflation
'increase' is the definition.
(inflation is a kind of increase)
'popular old means of decreasing not right for an' is the wordplay.
I cannot really understand how this works, but
'popular' could be 'in' ('in' can mean trendy) and 'in' is found within the answer.
'old' could be 'o' (common abbreviation eg in OE for Old English) and 'o' is located in the answer.
'right' could be 'flat' (synonyms) and 'flat' is found in the answer.
'an' could be 'i' (Roman numeral for one) and 'i' is present in the answer.
A single letter 'n' remains which might be clued in a way I don't understand.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for inflation that I've seen before include "Problem for savers" , "Economic problem" , "Building costs" , "General fall in value of money" , "Increasing costs" .)