Think about old touchpad alternative (5)
I believe the answer is:
mouse
'touchpad alternative' is the definition.
(I've seen this in another clue)
'think about old' is the wordplay.
'think' becomes 'muse' (to muse on something is to think about it).
'about' means one lot of letters goes inside another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'muse' enclosing 'o' is 'MOUSE'.
(Other definitions for mouse that I've seen before include "Rodent (for computing)" , "retiring person" , "Small creature" , "Timid creature" , "Small verminous rodent" .)