Endlessly regular, in his place (5)
I believe the answer is:
stead
'his place' is the definition.
'stead' can be an answer for 'place' (synonyms). I am not sure about the 'his' bit.
'endlessly regular' is the wordplay.
'endlessly' means to remove the last letter.
'regular' becomes 'steady' (similar in meaning).
'steady' with its last letter taken off is 'STEAD'.
'in' acts as a link.
Can you help me to learn more?
(Another definition for stead that I've seen is " Place, function or role of another".)