No official line given by America (7)
I believe the answer is:
refusal
'no' is the definition.
(refusing is saying no)
'official line given by america' is the wordplay.
'official' becomes 'ref' (match official in football, say).
'line' becomes 'l' (used when specifying particular lines from a poem).
'given by' means one lot of letters go next to another.
'america' becomes 'usa' (synonyms).
'l' after 'usa' is 'usal'.
'ref'+'usal'='REFUSAL'
(Other definitions for refusal that I've seen before include "Repudiation" , "Turning-down; denial" , "The act of saying no" , "Negation" , "Stop by fence" .)