Tower completely infested by nasty bug (7)
I believe the answer is:
tugboat
'tower' is the definition.
(I've seen this before)
'completely infested by nasty bug' is the wordplay.
'completely' becomes 'toat' ('to a T' can mean 'completely').
'infested by' means one lot of letters goes inside another.
'nasty' indicates an anagram.
'bug' is an anagram of 'ugb'.
'toat' placed around 'ugb' is 'TUGBOAT'.
(Other definitions for tugboat that I've seen before include "water tower" , "Drawing craft" , "Small vessel pulling a big one" , "Vessel drawing" , "Ship pulling a larger one" .)