Dogs are entering narrow gap on a climb (5)
I believe the answer is:
tails
'dogs' is the definition.
('tail' can be a synonym of 'dog')
'are entering narrow gap on a climb' is the wordplay.
'are' becomes 'a' (the 'are' is a metric unit of area).
'entering' indicates putting letters inside.
'narrow gap' becomes 'slit'.
'on a climb' says the letters should be written backwards.
'a' put inside 'slit' is 'sliat'.
'sliat' reversed gives 'TAILS'.
(Other definitions for tails that I've seen before include "Coin-tossing call" , "Ends" , "Follows closely" , "Evening dress; coin toss call" , "Animals' extremities" .)