One coming in bats on, making defence (7)
I believe the answer is:
bastion
'defence' is the definition.
(I know that bastion is a type of defence)
'one coming in bats on making' is the wordplay.
'one' becomes 'i' (Roman numeral).
'coming in' means one lot of letters goes inside another.
'making' indicates an anagram.
'bats'+'on'='batson'
'batson' is an anagram of 'baston'.
'i' put into 'baston' is 'BASTION'.
(Other definitions for bastion that I've seen before include "Obtains (anag.)" , "Fortified place near hostile territory" , "Stronghold, fortification" , "Obtains in fortified place or stronghold" , "Strong defender" .)