They ebb and flow in links either side of delta (5)
I believe the answer is:
tides
'they ebb and flow' is the definition.
(I've seen this before)
'links either side of delta' is the wordplay.
'links' becomes 'ties' ('tie' can be a synonym of 'link').
'either side of' is an insertion indicator (one word goes around another one).
'delta' becomes 'd' (phonetic alphabet: alpha, bravo, charlie etc.).
'ties' going around 'd' is 'TIDES'.
'in' is the link.
(Other definitions for tides that I've seen before include "drifts" , "Critical moments" , "Current trends" , "They occur daily" , "Oceanic movements" .)