Lava gives back-up to rodents (6)
I believe the answer is:
pumice
'lava' is the definition.
(I have seen 'Porous lava ' mean 'pumice' so perhaps 'lava' could also mean 'pumice')
'back-up to rodents' is the wordplay.
'back' says the letters should be written in reverse.
'to rodents' becomes 'mice' (mouse is a kind of rodent).
'up' back-to-front is 'pu'.
'pu'+'mice'='PUMICE'
'gives' is the link.
(Other definitions for pumice that I've seen before include "Light, porous volcanic rock" , "Mineral" , "Lava stone" , "Light form of solidified lava" , "Solidified lava used as an abrasive" .)