Leaves volunteers to capture Spain (3)
I believe the answer is:
tea
'leaves' is the definition.
(as in tea leaves)
'volunteers to capture spain' is the wordplay.
'volunteers' becomes 'TA' (abbreviation for Territorial Army).
'to capture' is an insertion indicator.
'spain' becomes 'e' (abbreviation for Espana).
'ta' enclosing 'e' is 'TEA'.
(Other definitions for tea that I've seen before include "Char" , "Infused beverage" , "Afternoon meal; drink" , "TREE" , "Plantation product" .)