Portable device providing about 50% more space (5)
I believe the answer is:
pager
'portable device' is the definition.
'about 50 more space' is the wordplay.
'about' shows that the letters should be reversed in order.
'50' suggests halving the letters.
'space' becomes 'gap' (gap is a kind of space**).
'more' halved is 're'.
're'+'gap'='regap'
'regap' reversed gives 'PAGER'.
'providing' is the link.
(Other definitions for pager that I've seen before include "Way to contact" , "One calling" , "Small radio bleeper for contact" , "This may summon one" , "Radio alert device" .)