Fly-in mail, trend used off and on with restrictions (7)

I believe the answer is:

limited

'with restrictions' is the definition.
(something limited is restricted)

'fly-in mail trend used off and on' is the wordplay.
'used off and on' means one should take alternating letters.
'fly'+'in'+'mail'+'trend'='flyinmailtrend'
The alternating letters of 'flyinmailtrend' are 'LIMITED'.

(Other definitions for limited that I've seen before include "Circumscribed, restricted" , "Confined within bounds" , "Checked" , "Of small range" , "Restricted in extent/size" .)

I've seen this clue in The Telegraph.

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