Top of big tree enveloped by single blaze (7)
I believe the answer is:
bonfire
'blaze' is the definition.
(I've seen this before)
'top of big tree enveloped by single' is the wordplay.
'top of' suggests taking the first letters.
'tree' becomes 'fir' (fir tree is a kind of tree).
'enveloped by' indicates putting letters inside.
'single' becomes 'one' (a single thing - only one).
The initial letter of 'big' is 'b'.
'fir' inserted into 'one' is 'onfire'.
'b'+'onfire'='BONFIRE'
(Other definitions for bonfire that I've seen before include "lit up in the garden" , "Source of blazing" , "Signal" , "It's usually lit on Guy Fawkes' day" , "It is lit to celebrate Guy Fawkes's day" .)