St Patrick initially harbouring English one in flight (4)
I believe the answer is:
step
'one in flight' is the definition.
(part of a flight of stairs)
'st patrick initially harbouring english' is the wordplay.
'initially' suggests taking the first letters.
'harbouring' means one lot of letters goes inside another.
'english' becomes 'e' (abbreviation).
The first letter of 'patrick' is 'p'.
'st'+'p'='stp'
'stp' going around 'e' is 'STEP'.
(Other definitions for step that I've seen before include "Pace - part of a procedure" , "Measure taken" , "A pace for pets" , "some progress" , "Second half of quick 20?" .)