Unrestrained sheep to breathe heavily (7)
I believe the answer is:
rampant
'unrestrained' is the definition.
(I've seen this before)
'sheep to breathe heavily' is the wordplay.
'sheep' becomes 'ram' (male sheep).
'to breathe heavily' becomes 'pant' (I've seen this in another clue).
'ram'+'pant'='RAMPANT'
(Other definitions for rampant that I've seen before include "Unchecked - standing up on hind legs" , "Unrestrained and violent" , "Unrestrained, as animal may be in heraldry" , "Out of control" , "On the rampage" .)