Firm line taken in balancing act (9)
I believe the answer is:
tightrope
'taken in balancing act' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'firm line' is the wordplay.
'firm' becomes 'tight' (synonyms).
'line' becomes 'rope' (rope is a kind of line).
'tight'+'rope'='TIGHTROPE'
Can you help me to learn more?
(Other definitions for tightrope that I've seen before include "Difficult thing to walk on" , "Acrobats perform on it, usually with safety net" , "- - -walker" , "Cable on which acrobatics are performed" , "High wire" .)