Chain that's loose if back to front? (4)
I believe the answer is:
reef
'chain' is the definition.
(I have seen 'Chain of rocks ' mean 'reef' so perhaps 'chain' could also mean 'reef')
'loose if back to front?' is the wordplay.
'back' suggests the final letters (the letter at the back).
'to' is a charade indicator (letters next to each other) (I've seen this in other clues).
'front?' becomes 're' (I can't justify this - if you can you should give a lot more credence to this answer).
The last letters of 'loose if' are 'ef'.
'ef' put after 're' is 'REEF'.
'that's' acts as a link.
(Other definitions for reef that I've seen before include "Submerged rocks" , "sailors consider dangerous" , "Kind of knot (avoided by sailors?)" , "Coral structure" , "endangered habitat" .)