President once with aspiration for boost (4)
I believe the answer is:
hike
'for boost' is the definition.
(synonyms)
'president once with aspiration' is the wordplay.
'president once' becomes 'ike' (nickname of Dwight Eisenhower).
'with' means one lot of letters go next to another.
'aspiration' becomes 'h' (H represents an aspirated sound).
'ike' after 'h' is 'HIKE'.
(Other definitions for hike that I've seen before include "(Price) increase; walk" , "Walk for pleasure in the country" , "(Go for) a long walk" , "Walking tour" , "Long ramble" .)