The right first measure almost all is relating to temperature (7)
I believe the answer is:
thermal
'relating to temperature' is the definition.
I can't tell whether this defines the answer.
'the right first measure almost all' is the wordplay.
'right' becomes 'r' (common abbreviation).
'first measure' becomes 'm' (1st letter of 'measure').
'almost' means to remove the last letter (most of the word but not all of it).
'all' with its last letter removed is 'al'.
'the'+'r'+'m'+'al'='THERMAL'
'is' is the link.
Can you help me to learn more?
(Other definitions for thermal that I've seen before include "Caloric" , "Upward current of warm air" , "Warm like winter woollies" , "A lot of hot air" , "Related to heat" .)