Sort of ferret about behind mast, getting tense (7)
I believe the answer is:
polecat
'sort of ferret' is the definition.
(a ferret is a domesticated polecat)
'about behind mast getting tense' is the wordplay.
'about' becomes 'ca' (abbreviation for 'circa').
'behind' means one lot of letters go next to another.
'mast' becomes 'pole'.
'getting' is a charade indicator (letters next to each other).
'tense' becomes 't' (abbreviation used in many dictionaries).
'ca' put after 'pole' is 'poleca'.
'poleca'+'t'='POLECAT'
(Other definitions for polecat that I've seen before include "Smelly American mammal" , "Unpleasant-smelling woodland animal" , "Weasel-like creature" , "Weasel-like mammal" , "Smelly animal" .)