Record tigers being let loose on railway (8)
I believe the answer is:
registry
'record' is the definition.
(I have seen 'official record' mean 'registry' so perhaps 'record' could also mean 'registry')
'tigers being let loose on railway' is the wordplay.
'being let loose' indicates an anagram.
'on' means one lot of letters goes inside another.
'railway' becomes 'ry' (abbreviation for railway).
'tigers' anagrammed gives 'egistr'.
'egistr' going inside 'ry' is 'REGISTRY'.
(Other definitions for registry that I've seen before include "record-holder" , "Place of record" , "Place where written documents or records are kept" , "Office where records are kept" , "where files may be observed" .)