Tunnellers sent back across Germany rarely (6)
I believe the answer is:
seldom
'rarely' is the definition.
(synonyms)
'tunnellers sent back across germany' is the wordplay.
'tunnellers' becomes 'moles' (animals which tunnel).
'sent back' is a reversal indicator.
'across' means one lot of letters goes inside another.
'germany' becomes 'd' (vehicle registration code for Deutschland).
'moles' written backwards gives 'selom'.
'selom' placed around 'd' is 'SELDOM'.
(Other definitions for seldom that I've seen before include "Models (anag)" , "Alluvium" , "on rare occasions" , "Only occasionally" , "Not often, infrequently" .)