With financial support, boy volunteers to back nags (5,2,2)

'nags' is the definition.

'with financial support boy volunteers to back' is the wordplay.
'with' means one lot of letters go next to another.
'financial support' becomes 'keep' (as in 'earn one's keep').
'boy' becomes 'son' ('son' can be a synonym of 'boy'**).
'volunteers' becomes 'ta' (abbreviation for Territorial Army).
'to back' shows that the letters should be reversed in order.
'ta' back-to-front is 'at'.
'keep'+'son' is 'keepson'.
'keepson'+'at'='KEEPS ON AT'