Current measures to protect the earth I designed for combatting areas with crowds (13)

'combatting areas with crowds' is the definition.
(places where large crowds once watched fighting)

'current measures to protect the earth i designed' is the wordplay.
'current measures' becomes 'amps' (units of electrical current).
'to protect' indicates putting letters inside.
'designed' indicates an anagram (designed with the letters).
'the'+'earth'+'i'='theearthi'
'theearthi' anagrammed gives 'hitheatre'.
'amps' going around 'hitheatre' is 'AMPHITHEATRES'.

'for' acts as a link.