Deer starts to notice all the excitement -- be still! (8)
I believe the answer is:
stagnate
'be still' is the definition.
(thesaurus)
'deer starts to notice all the excitement' is the wordplay.
'deer' becomes 'stag' (stag is a kind of deer).
'starts to' says to take the initial letters.
The first letters of 'notice all the excitement' is 'nate'.
'stag'+'nate'='STAGNATE'
(Other definitions for stagnate that I've seen before include "Become stale from lack of motion" , "Tan gates could vegetate" , "Become inactive (and malodorous)" , "Become dull through inactivity" , "Cease activity" .)