Tennis big guns periodically flag (6)
I believe the answer is:
ensign
'flag' is the definition.
(synonyms)
'tennis big guns periodically' is the wordplay.
'periodically' means one should take alternating letters (letters must be selected 'every so often').
'tennis'+'big'+'guns'='tennisbigguns'
The alternate letters of 'tennisbigguns' are 'ENSIGN'.
(Other definitions for ensign that I've seen before include "Badge, flag" , "SECOND PART OF 10" , "Lowly naval officer" , "Soldier who carries the Colours" , "Low-ranking naval officer" .)