Sadly, owls nearly starve without primarily eating rodents (5,5)

'rodents' is the definition.
(water vole is a kind of rodent)

'sadly owls nearly starve without primarily eating' is the wordplay.
'sadly' indicates an anagram.
'nearly' means to remove the last letter.
'without' means one lot of letters goes inside another ('without' can be similar in meaning to 'outside').
'primarily' indicates taking the first letters.
The initial letter of 'eating' is 'e'.
'owls' with its final letter taken away is 'owl'.
'owl'+'starve'='owlstarve'
'owlstarve' is an anagram of 'watervols'.
'watervols' placed around 'e' is 'WATER-VOLES'.