Rag time ally abroad welcomes, giving a wave (7)
I believe the answer is:
tsunami
'a wave' is the definition.
(tsunami is a kind of wave)
'rag time ally abroad welcomes' is the wordplay.
'rag' becomes 'sun' (I've seen this before).
'time' becomes 't' (abbreviation).
'ally abroad' becomes 'ami' ('friend' in French).
'welcomes' means one lot of letters goes inside another.
't'+'ami'='tami'
'sun' placed into 'tami' is 'TSUNAMI'.
'giving' acts as a link.
(Other definitions for tsunami that I've seen before include "wall of water" , "Huge tidal wave that causes great damage and loss of life" , "I am nuts! (anag)" , "High sea wave" , "Large sea wave caused by an earthquake" .)