Emergency call very singular (3)
I believe the answer is:
sos
'emergency call' is the definition.
(an SOS message sent in an emergency)
'very singular' is the wordplay.
'very' becomes 'so' (both can mean 'extremely').
'singular' becomes 's' (abbreviation sometimes used in dictionaries).
'so'+'s'='SOS'
(Other definitions for sos that I've seen before include "Help initially, Mr Morse" , "Help code in 13 Across" , "request for aid" , "plea" , "Appeal for help" .)