Starts to stagnate in this place (3)
I believe the answer is:
sit
'place' is the definition.
(sitting is a kind of placing)
'starts to stagnate in this' is the wordplay.
'starts to' suggests taking the first letters.
The initial letters of 'stagnate in this' is 'SIT'.
(Other definitions for sit that I've seen before include "To take the test" , "Perch" , "Rest on one's haunches" , "as magistrates do" , "Take a seat; pose" .)