Swimmer in Ghasri periodically (3)
I believe the answer is:
gar
'swimmer' is the definition.
(type of fish)
'ghasri periodically' is the wordplay.
'periodically' indicates alternate letters (letters must be selected 'every so often').
The alternate letters of 'ghasri' are 'GAR'.
'in' is the link.
(Other definitions for gar that I've seen before include "Type of fish" , "A fish; cloth scrap (reversed)" , "swimmer" , "Pike-like fish" , "something fishy here" .)