Study at four in place of worship, deep in thought (13)
I believe the answer is:
contemplative
'deep in thought' is the definition.
'contemplative' can be an answer for 'thought' (thesaurus). I'm not sure about the 'deep in' bit.
'study at four in place of worship' is the wordplay.
'study' becomes 'con' (con can mean to study).
'four' becomes 'IV' (Roman numerals for four).
'in' indicates putting letters inside.
'place of worship' becomes 'temple' (temple is a kind of place of worship).
'at'+'iv'='ativ'
'ativ' put within 'temple' is 'templative'.
'con'+'templative'='CONTEMPLATIVE'
Can you help me to learn more?
(Other definitions for contemplative that I've seen before include "lost in thought" , "Employing meditation" , "Thoughtful" .)