Lofty, aggressive, gutless Den's back (6)
I believe the answer is:
aerial
'lofty' is the definition.
(I've seen this in another clue)
'aggressive gutless den's back' is the wordplay.
'gutless' means to remove the middle letters.
'den' becomes 'lair' (synonyms).
'back' says the letters should be written backwards.
'aggressive' with its middle taken out is 'ae'.
'lair' in reverse letter order is 'rial'.
'ae'+'rial'='AERIAL'
(Other definitions for aerial that I've seen before include "Antenna for capturing radio or TV signals" , "Radio waves receiver" , "It picks up radio waves" , "Operating aloft" , "Ethereal" .)