See you mostly pay attention to Spanish river after beginning to cross (7)
I believe the answer is:
cheerio
'see you' is the definition.
(I've seen this before)
'mostly pay attention to spanish river after beginning to cross' is the wordplay.
'mostly' means to remove the last letter.
'pay attention to' becomes 'heed' (I've seen this before).
'spanish river' becomes 'rio' (I've seen this before).
'after' is a charade indicator (letters next to each other).
'beginning to' indicates taking the first letters.
The first letter of 'cross' is 'c'.
'heed' with its last letter taken off is 'hee'.
'hee'+'rio'='heerio'
'heerio' after 'c' is 'CHEERIO'.
(Other definitions for cheerio that I've seen before include "Bye" , "See you" , "A final word" , "A hearty farewell, begins with applause" , "good health" .)