Call individual after observing degree of acidity (5)
I believe the answer is:
phone
'call' is the definition.
(phone can mean to call by telephone)
'individual after observing degree of acidity' is the wordplay.
I cannot quite understand how this works, but
'individual' could be 'one' (an individual is one person) and 'one' is found within the answer.
'acidity' could be 'ph' and 'ph' is found within the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for phone that I've seen before include "Communications device" , "Covert monitoring of electronic exchanges" , "Call (someone)" , "Ring (someone)" , "Make a call" .)