Prisoner starts to tunnel after getting into pipe (7)
I believe the answer is:
hostage
'prisoner' is the definition.
(hostage is a kind of prisoner)
'starts to tunnel after getting into pipe' is the wordplay.
'starts to' says to take the initial letters.
'into' means one lot of letters goes inside another.
'pipe' becomes 'hose' (I've seen this before**).
The first letters of 'tunnel after getting' is 'tag'.
'tag' going inside 'hose' is 'HOSTAGE'.
(Other definitions for hostage that I've seen before include "One held captive" , "She-goat (anag)" , "Detainee - gash toe (anag)" , "One caught" , "Captive held for ransom" .)