Handheld device that indicates high acidity? (5)
I believe the answer is:
phone
This might be a double definition.
'handheld' is the first definition.
(I know that phone is a type of handheld)
'device that indicates high acidity?' is the second definition.
'phone' can be an answer for 'device' (phone is a kind of device). I'm not certain of the rest of the definition.
(Other definitions for phone that I've seen before include "Call; eg, mobile" , "Short piece of electronic equipment" , "Covert monitoring of electronic exchanges" , "Call, ring" , "Make a call" .)