Surrounding temperature supports the last word about peaks in British Isles (7)
I believe the answer is:
ambient
'surrounding' is the definition.
(synonyms)
'temperature supports the last word about peaks in british isles' is the wordplay.
'temperature' becomes 't' (abbreviation).
'supports' means one lot of letters go next to another (bottom letters provide a support for the top ones).
'the last word' becomes 'amen' (last word in a prayer).
'about' means one lot of letters goes inside another.
'peaks in' indicates taking the first letters (I've seen 'peak in' mean this (in a down clue, the letter at the top of the word)).
The first letters of 'british isles' is 'bi'.
'amen' placed around 'bi' is 'ambien'.
't' put after 'ambien' is 'AMBIENT'.
(Other definitions for ambient that I've seen before include "Surrounding, on all sides - light or temperature" , "Related to the immediate surroundings" , "Surrounding or encompassing, say temperature" , "Of music, say, played at low volume to create atmosphere" , "Surrounding - mean bit (anag)" .)