Test in term: is able to appear regularly (5)
I believe the answer is:
trial
'test' is the definition.
(synonyms)
'term is able to appear regularly' is the wordplay.
'to appear regularly' indicates alternate letters (I've seen 'appearing regularly' mean this).
'term'+'is'+'able'='termisable'
The alternating letters of 'termisable' are 'TRIAL'.
'in' acts as a link.
(Other definitions for trial that I've seen before include "Pilot" , "Testing, experiment" , "Test to determine guilt or innocence" , "competition" , "Troublesome thing; test" .)