Royal couple put their foot in it (3)
I believe the answer is:
err
'their foot in it' is the definition.
I can't tell whether this defines the answer.
'royal couple' is the wordplay.
'royal couple' means two substitutions for 'royal'.
'royal' becomes 'er' (abbreviation for Elizabeth Regina).
'royal' becomes 'r' (abbreviation).
'er'+'r'='ERR'
'put' is the link.
Can you help me to learn more?
(Other definitions for err that I've seen before include "Go wrong" , "Make a wrong move" , "'To . . . is human, to forgive divine'" , "do something you shouldn't" , "Be mistaken" .)