Overwhelming force gripping North America, I have to retreat (7)
I believe the answer is:
tsunami
'overwhelming force' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'gripping north america i have to retreat' is the wordplay.
'gripping' indicates putting letters inside.
'north america' becomes 'na'.
'have to' becomes 'must' (I've seen this before).
'retreat' says the letters should be written backwards.
'i'+'must'='imust'
'imust' written backwards gives 'tsumi'.
'na' put into 'tsumi' is 'TSUNAMI'.
Can you help me to learn more?
(Other definitions for tsunami that I've seen before include "Large sea wave caused by an earthquake" , ""Harbour wave"" , "High sea wave" , "Strong surge" , "Large ocean wave" .)