Impressions not on board to press down tightly (4)
I believe the answer is:
tamp
'press down tightly' is the definition.
(I've seen this before)
'impressions not on board' is the wordplay.
'impressions' becomes 'stamps' ('stamp' can be a synonym of 'impression').
'not' is a deletion indicator.
'on board' becomes 'ss' (I've seen this in other clues).
'stamps' with 'ss' taken out is 'TAMP'.
'to' acts as a link.
(Other definitions for tamp that I've seen before include "Firmly ram down" , "Press, pack (down)" , "Compact" , "Pack (eg earth) down firmly" , "Push down tobacco" .)