Very advanced phone ultimately beset by snag (2-4)
I believe the answer is:
hi-tech
'very advanced' is the definition.
(using advanced technology)
'phone ultimately beset by snag' is the wordplay.
'ultimately' indicates one should take the final letters.
'beset by' is an insertion indicator ('beset' could historically mean 'cover').
'snag' becomes 'hitch' (synonyms).
The final letter of 'phone' is 'e'.
'e' going into 'hitch' is 'HI-TECH'.
(Other definitions for hi-tech that I've seen before include "Style of interior design using features of industrial equipment" , "Involving the latest electronic gadgetry" , "Using the latest science" , "(Using) the latest developments" , "Sophisticated electronics" .)