Not answering mobile so unnerves one holding power (12)
I believe the answer is:
unresponsive
'not answering' is the definition.
(I've seen this before)
'mobile so unnerves one holding power' is the wordplay.
'mobile' is an anagram indicator.
'one' becomes 'i' (Roman numeral).
'holding' indicates putting letters inside.
'power' becomes 'p'.
'so'+'unnerves'+'i'='sounnervesi'
'sounnervesi' with letters rearranged gives 'unresonsive'.
'unresonsive' going around 'p' is 'UNRESPONSIVE'.
(Other definitions for unresponsive that I've seen before include "disengaged" , "Not reacting to stimuli" , "Not answering" .)