Former policemen surrounding two badly fortified houses (4-6)

'fortified houses' is the definition.
(type of defensive structure)

'former policemen surrounding two badly' is the wordplay.
'former policemen' becomes 'peelers' (historical nickname for policemen).
'surrounding' means one lot of letters goes inside another.
'badly' indicates an anagram (letters in the wrong order).
'two' anagrammed gives 'tow'.
'peelers' going around 'tow' is 'PEEL-TOWERS'.