I taxi over after heading to Ann Summers (5)
I believe the answer is:
abaci
'summers' is the definition.
(I've seen this in another clue)
'i taxi over after heading to ann' is the wordplay.
'taxi' becomes 'cab' (synonyms).
'over' is a reversal indicator.
'after' means one lot of letters go next to another.
'heading to' says to take the initial letters (the head of the word).
The first letter of 'ann' is 'a'.
'i'+'cab'='icab'
'icab' in reverse letter order is 'baci'.
'baci' after 'a' is 'ABACI'.
(Other definitions for abaci that I've seen before include "Counting devices" , "Early computers" , "You can count on them" , "Counting-frames" , "They can be counted on" .)