Impressive tango by pianist who's lost weight (7)
I believe the answer is:
telling
'impressive' is the definition.
(I know that impressive can be written as telling)
'tango by pianist who's lost weight' is the wordplay.
'tango' becomes 'T' (phonetic alphabet: alpha, bravo, charlie etc.).
'by' says to put letters next to each other.
'pianist' becomes 'ellington' (jazz pianist Duke Ellington).
'who's lost' is a deletion indicator.
'weight' becomes 'ton' (unit of weight).
'ellington' with 'ton' removed is 'elling'.
't'+'elling'='TELLING'
(Other definitions for telling that I've seen before include "Powerful; relating" , "Counting" , "influential" , "Powerful - effective" , "Powerful; informing" .)