Have third thoughts about defensive structure? (7)
I believe the answer is:
redoubt
'defensive structure?' is the definition.
(I know that Redoubt is a type of Fortification)
'have third thoughts about' is the wordplay.
I cannot really see how this works, but
'about' could be 're' ('re' means regarding or about) and 're' is found within the answer.
'third thoughts' could be 'o' (3rd letter of 'thoughts') and 'o' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for redoubt that I've seen before include "Tube rod used in small fortification" , "Temporary or outward fortification" , "Fieldwork defending a pass" , "Defensive outwork" , "Fortified stronghold" .)