Devotion that is within quiet and extremes of tranquillity (5)
I believe the answer is:
piety
'devotion' is the definition.
(I've seen this in another clue)
'that is within quiet and extremes of tranquillity' is the wordplay.
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'within' means one lot of letters goes inside another.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'and' says to put letters next to each other.
'extremes of' suggests removing the centre (only the letters at the extremes of the word).
'tranquillity' with its centre removed is 'ty'.
'p'+'ty'='pty'
'ie' placed within 'pty' is 'PIETY'.
(Other definitions for piety that I've seen before include "Holiness" , "Religious feeling" , "Attentive religious practice" , "Quality of devoutness" , "Devotion to religious principles" .)