Instrument seen in Frankfurt U-Bahn (4)
I believe the answer is:
tuba
'instrument' is the definition.
(tuba is a kind of instrument)
'in frankfurt u-bahn' is the wordplay.
'in' indicates a hidden word.
'TUBA' is hidden within 'frankfurt u bahn'.
'seen' acts as a link.
(Other definitions for tuba that I've seen before include "Musical instrument -- 5 (rev)" , "Producer of note" , "One's blown" , "Brass instrument of low pitch" , "It's not top brass" .)