Spreading unchecked (7)
I believe the answer is:
'spreading unchecked' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for rampant that I've seen before include "Unbridled" , "Unchecked (and violent?)" , "Unchecked - standing up on hind legs" , "Unrestrained, flourishing excessively" , "Growing or spreading uncontrollably" .)