Crossword Genius

Dog's on master's lead for a fixed period (3,4)

Ross

I believe the answer is:

set term

'a fixed period' is the definition.
(I've seen this in another clue)

'dog's on master's lead' is the wordplay.
'dog' becomes 'setter' (setter is a type of dog).
'on' means one lot of letters go next to another.
'lead' indicates taking the first letters.
The initial letter of 'masters' is 'm'.
'setter'+'m'='SET TERM'

'for' is the link.

I've seen this clue in The Guardian.
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