Breathes with different rhythm ultimately -- indication of temperature (8)
I believe the answer is:
isotherm
'indication of temperature' is the definition.
'breathes with different rhythm ultimately' is the wordplay.
'breathes' becomes 'is' (I've seen this in other clues).
'with' is a charade indicator (letters next to each other).
'different' becomes 'other' ('other' can be similar in meaning to 'different').
'ultimately' suggests the final letters.
The final letter of 'rhythm' is 'm'.
'is'+'other'+'m'='ISOTHERM'
(Other definitions for isotherm that I've seen before include "Line joining equally hot spots" , "Contour line of equal temperature" , "Line joining places of equal temperature" , "that's charted to a degree" , "Curve of equal temperatures" .)