Cross any two letters inside (5)
I believe the answer is:
angry
'cross' is the definition.
(I've seen this in another clue)
'any two letters inside' is the wordplay.
'two letters' means two substitutions for 'letter'.
'letter' becomes 'g'.
'letter' becomes 'r' ().
'inside' indicates putting letters inside.
'g'+'r'='gr'
'any' enclosing 'gr' is 'ANGRY'.
(Other definitions for angry that I've seen before include "Cross" , "Vexed" , "Upset" , "Inflamed" , "Irate, mad" .)