Muscly tongue, swearing (6,8)
I believe the answer is:
strong language
'swearing' is the definition.
(I've seen this before)
'muscly tongue' is the wordplay.
'muscly' becomes 'strong' (I've seen this in another clue).
'tongue' becomes 'language' (I've seen this before).
'strong'+'language'='STRONG LANGUAGE'
(Other definitions for strong language that I've seen before include "something trooper might use?" , "Swearing" .)