One on the fiddle from prison is tense (7)
I believe the answer is:
cellist
'one on the fiddle' is the definition.
(someone playing a fiddle-like instrument)
'prison is tense' is the wordplay.
'prison' becomes 'cell' (prison cell).
'tense' becomes 't' (abbreviation used in many dictionaries).
'cell'+'is'+'t'='CELLIST'
'from' is the link.
(Other definitions for cellist that I've seen before include "Member of orchestra" , "Musician in string section" , "E.g. Casals, du Pre" , "String player" , "Du Pre or Lloyd Weber" .)