A tap losing pressure -- reach with some difficulty (2,1,7)
I believe the answer is:
at a stretch
'with some difficulty' is the definition.
(at a stretch means in difficult circumstances)
'a tap losing pressure reach' is the wordplay.
'losing' indicates named letters should be taken away.
'pressure' becomes 'p' (abbreviation).
'reach' becomes 'stretch' (synonyms).
'tap' with 'p' taken out is 'ta'.
'a'+'ta'+'stretch'='AT A STRETCH'
(Other definitions for at a stretch that I've seen before include "With much effort" , "Running, possibly" , "in one continuous period" , "With difficulty; without interruption" .)