Test, and test regularly, Catholic's composition (5)
I believe the answer is:
motet
'catholic's composition' is the definition.
'motet' can be an answer for 'composition' (I've seen this before). I am not certain of the 'catholic's' bit.
'test and test regularly' is the wordplay.
'test' becomes 'mot'.
'and' means one lot of letters go next to another.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
The alternate letters of 'test' are 'et'.
'mot'+'et'='MOTET'
Can you help me to learn more?
(Other definitions for motet that I've seen before include "Piece for choir" , "Short sacred choral work" , "Religious choral composition" , "Unaccompanied choral composition with sacred lyrics" , "Certain one is morally superior" .)