INSTR returns the position in string1 of string2, e.g.
SUBSTR returns the substring of string1 starting at position m, n characters long (or until the end of the string if length is not specified); e.g:
returns 'X'. Therefore you just need
SELECT SUBSTR(desc_id,INSTR(desc_id,'->') +2) FROM tab_test;
There are more options with INSTR and SUBSTR, e.g. to find the nth occurrence instead of just the first one, SUBSTR can work backwards from the end etc. The SUBSTRB, SUBSTRC etc variations are to support multi-byte character sets.