Lose track of rodents crossing mountains, going up? (8)
I believe the answer is:
misplace
'lose track of' is the definition.
'rodents crossing mountains going up?' is the wordplay.
'rodents' becomes 'mice' (mouse is a kind of rodent).
'crossing' means one lot of letters goes inside another.
'mountains' becomes 'alps' (alp is a kind of mountain).
'going up?' says the letters should be written in reverse.
'alps' in reverse letter order is 'spla'.
'mice' placed around 'spla' is 'MISPLACE'.
(Other definitions for misplace that I've seen before include "Be unable to find" , "Lose - limp case (anag)" , "Lose temporarily - put in wrong location" , "Wrongly put - lose" , "Do bad filing" .)