Let me in Rough Crossing - a small space for lead perhaps? (7)
I believe the answer is:
element
'lead perhaps?' is the definition.
(lead is a kind of element)
'let me in rough crossing a small space' is the wordplay.
'in rough' indicates anagramming the letters.
'crossing' is an insertion indicator (to cross can mean to go over or around).
'a small space' becomes 'en' (in printing, a space as wide as a letter 'n').
'let'+'me'='letme'
'letme' is an anagram of 'elemt'.
'elemt' placed around 'en' is 'ELEMENT'.
'for' acts as a link.
(Other definitions for element that I've seen before include "Basic constituent; aspect" , "Trace" , "Constituent part (7)" , "Eg, one dn" , "eg Fire, air" .)