Religious believers curtailed underwear thefts (10)
I believe the answer is:
pantheists
'religious believers' is the definition.
'pantheists' can be an answer for 'believers' (pantheist is a kind of believer). I am not certain of the 'religious' bit.
'curtailed underwear thefts' is the wordplay.
'curtailed underwear' becomes 'pant' (I am not sure about this - if you are sure you should believe this answer much more).
'thefts' becomes 'heists' (heist is a kind of theft).
'pant'+'heists'='PANTHEISTS'
Can you help me to learn more?
(Another definition for pantheists that I've seen is " religious folk".)