Free of trouble on top of fell (12)
I believe the answer is:
unrestrained
'free' is the definition.
(associated in meaning)
'trouble on top of fell' is the wordplay.
'trouble' becomes 'unrest' (I've seen this before).
'on top of' means one lot of letters go next to another.
'fell' becomes 'rained' (as in rain falling).
'unrest'+'rained'='UNRESTRAINED'
'of' acts as a link.
(Other definitions for unrestrained that I've seen before include "Not restricted" , "Not held back" , "Wild" , "spreading everywhere?" , "Unchecked" .)