Keep blocking Thirty Acts periodically in retribution (3,3,3)
I believe the answer is:
tit for tat
'retribution' is the definition.
(I've seen this in another clue)
'keep blocking thirty acts periodically' is the wordplay.
'keep' becomes 'fort' (I've seen this in other clues).
'blocking' is an insertion indicator (to block can mean to plug a gap in something).
'periodically' means one should take alternating letters (letters must be selected 'every so often').
'thirty'+'acts'='thirtyacts'
The alternating letters of 'thirtyacts' are 'titat'.
'fort' inserted into 'titat' is 'TIT-FOR-TAT'.
'in' acts as a link.
(Other definitions for tit for tat that I've seen before include "In annoyance, return" , "Appropriate retaliation" , "Counterattack" , "Retaliation for an injury - almost equal" , "revenge" .)