Unfriendly dog maybe primarily guarding oarsman crossing lake (7)
I believe the answer is:
growler
'unfriendly dog maybe' is the definition.
'primarily guarding oarsman crossing lake' is the wordplay.
'primarily' says to take the initial letters.
'oarsman' becomes 'rower' ('rower' can be a synonym of 'oarsman').
'crossing' means one lot of letters goes inside another.
'lake' becomes 'l'.
The first letter of 'guarding' is 'g'.
'rower' going around 'l' is 'rowler'.
'g'+'rowler'='GROWLER'
(Other definitions for growler that I've seen before include "Iceberg" , "Dog perhaps" , "Perhaps bear" , "Unfriendly dog" .)